How to Connect a SIP Trunk to a Hosted PBX: Setup Checklist

Tom Reed · Updated 2 November 2026

To connect a SIP trunk to a hosted PBX, add the trunk in the admin portal using the authentication your carrier supports (IP or username and password), give each inbound number a route to a person, group, queue or call flow, and set each user's outbound caller ID from your connected numbers. Check inbound and outbound calls before you move live traffic. For an MSP or reseller it's the same steps per customer, inside each customer's own project, which is how the SessionPBX partner programme is set up.

This checklist is for MSPs, ITSPs, VoIP providers and IT resellers connecting a customer's SIP trunk to SessionPBX, and for the in-house admin doing the SIP trunk setup themselves. It assumes you already know what a trunk is; if not, start with our beginner's guide to SIP trunking and come back. SessionPBX is a bring-your-own SIP trunk phone system: the trunk is the customer's own, from any SIP-compliant carrier they choose, and the carrier bills them directly for numbers and calls. A quick naming note: Teams Phone and Teams Business are SessionPBX plans, not Microsoft Teams. SessionPBX is a standalone phone system that can be hosted by SessionTalk or self-hosted on a partner's or customer's own servers.

What do you need from the carrier before you start?

Before you touch the admin portal, collect everything the carrier side of the connection needs. Having it in one place makes the SIP trunk setup a short, tidy job rather than a back-and-forth with the carrier's support desk.

• An active account with a SIP-compliant carrier. If the customer doesn't have a carrier yet, read how to choose a SIP trunk provider for a hosted PBX first. This checklist doesn't rank or recommend carriers; it works the same whichever one the customer picks.
• The list of numbers on the trunk. Write down every inbound number the customer uses and what each one is for, such as the main line, a sales line or a support line. You'll need this list when you build the inbound routes.
• How the carrier authenticates. Some carriers recognise your system by its IP address, others give you a username and password for the trunk. Ask which they prefer, and get the trunk details from the carrier's own portal or documentation.
• Enough call capacity. Make sure the trunk can carry the number of calls the customer makes at once. How many SIP channels you need walks through the sizing.

The trunk, the numbers on it and the call charges all stay between the customer and their carrier. SessionPBX connects to that trunk; it doesn't supply it.

IP authentication or credentials: which should you use?

There are two common ways a carrier knows that calls are coming from your phone system:

• IP authentication. The carrier trusts traffic from a known IP address. There's no trunk password to store, but the carrier needs the right address on file.
• Credential authentication (registration). Your phone system registers with the carrier using a username and password, much like a softphone signs in to a phone system.

The right choice is whichever the carrier prefers. SessionPBX lets you register with your carrier by IP or by credentials, so you don't have to fit the carrier around the phone system. If you use credentials, the trunk usernames and passwords are stored encrypted at rest with AES-256-GCM, and SIP signalling, messaging and the admin portal use TLS by default. IP access controls and automated blocking help protect the system against repeated login attempts, suspicious traffic and toll fraud.

Carrier-side settings vary, so follow the carrier's own instructions for their end of the connection rather than copying steps from another provider.

How do you route inbound numbers on a PBX SIP trunk?

Once the trunk is connected, every inbound number needs somewhere to go. On a PBX SIP trunk, this is where most of the planning pays off. Go back to your list of numbers and decide, for each one, who or what should answer.

In SessionPBX you give each inbound number its own route to a person, group, queue or call flow. You can upload or record greetings and announcements, so callers on the sales line and callers on the support line can hear different welcomes and clear instructions.

A simple plan for a small customer might look like this:

• Main number goes to a call flow with a short menu, or straight to reception.
• Sales number goes to a ring group of the sales team.
• Support number goes to a queue so callers wait in order.
• Direct numbers go to the named person.

Ring groups, call queues and visual call flows with IVR and business hours are part of the Teams Business plan. Teams Phone covers everyday calling, messaging, voicemail, team contacts and app provisioning, with SIP trunks for external calls, so on that plan inbound numbers route to people. Either way, set opening hours and an out-of-hours route where the plan supports it, so calls after closing reach an announcement, voicemail or another destination instead of ringing out.

How do you set outbound caller ID?

Outbound calls go out over the same trunk, so callers see whichever number you present. In SessionPBX you set each member's outbound caller ID from your connected numbers. Decide this per person:

• Sales staff might present the sales number so callbacks reach the team.
• Support staff might present the support number.
• Named staff with their own direct number might present that.

Only present numbers that are on the customer's trunk. If a carrier has rules about which numbers can be presented, those rules are set by the carrier, so check with them. For a deeper look at the choices, see how to set outbound caller ID on a business phone system.

How should you check the trunk before cutover?

A common mistake is to call a colleague, hear a clear call and assume the trunk works. Internal calls between team members stay on the platform and don't use the trunk at all, so they prove the apps work, not the carrier connection. Only external calls go over the SIP trunk.

Check these instead, before any live traffic moves:

• Inbound to every number. Call each number on the trunk from an outside phone and confirm it reaches the right person, group, queue or call flow, and that the right greeting plays.
• Outbound from a few users. Call an outside phone and confirm the caller ID shows the number you set.
• Voicemail. Leave a message on a user who doesn't answer and confirm it arrives, in the app or by email with the recording attached if that's switched on.
• Out of hours. If the customer has opening hours, check the closed route too.

If calls connect but there's no audio in one direction, or calls drop, check the customer's network first; the VoIP firewall checklist covers what to allow. If a user's app won't sign in at all, that's a registration problem rather than a trunk problem, and the registration failed troubleshooting guide explains the usual error codes.

Users don't need to type SIP settings into their apps. Invite them by email, and they set up mobile apps with a QR code or desktop apps with a setup link. If anyone asks what those settings would be, SIP softphone account settings explained covers them. You can point users to the apps they'll run once the trunk is live, on Windows, macOS, iOS and Android.

When every check passes, cutover is the carrier's step: the carrier points the numbers at the new system, or completes the number port if the numbers are coming from somewhere else.

SIP trunk setup checklist

Use this table per customer. It's vendor-neutral, so it works with any SIP-compliant carrier, whether the customer is on hosted or self-hosted SessionPBX.

How do MSPs handle trunks across many customers?

For an MSP or reseller, connecting trunks is not a one-off job; it's a repeatable step in every customer rollout. SessionPBX keeps it tidy by giving each customer their own SessionPBX project under your partner account. You create it, you own it, and you manage extensions, numbers, the SIP trunk and call flows for that customer.

That means each customer can use a different carrier without affecting anyone else. One customer might stay on the carrier they already have, while another moves to the carrier you work with most. The carrier bills the customer directly for numbers and calls, and there's no per-minute markup: you can switch trunks in the admin portal whenever the customer's rates change. For the wider rollout, from the first conversation to go-live, see the MSP customer onboarding runbook.

You also choose where each customer runs. Put customers on a hosted PBX that works with your own carrier, hosted in the UK (London), the US (New York) or Singapore, with SessionTalk handling hosting and maintenance. Or run SessionPBX on your own servers: self-hosted is available to approved partners only, with many customer tenants on your own server and every customer kept separate. The trunk steps in this checklist are the same either way.

As a partner you set your own price and invoice your customer, and you can put your logo and in-app product name on the softphones your customers use with app logo branding. If that sounds like the way you want to sell phone systems, become a SessionPBX partner: you don't need a portal account to apply, and every application is reviewed by our team.

Connecting a SIP trunk comes down to a few clear decisions: how the carrier authenticates, where each number goes, what each person presents on outbound calls, and what you check before cutover. Do those the same way for every customer and trunks stop being the risky part of a rollout. When you're ready to run that process under your own name, apply to the SessionPBX partner programme.

FAQ

Can I use any SIP trunk with SessionPBX?

Yes, as long as it comes from a SIP-compliant carrier. You can register with the carrier by IP or by credentials, whichever the carrier supports, and stored trunk credentials are encrypted at rest. SessionPBX doesn't tie you to one carrier, so the customer can keep the trunk they already have or bring a new one.

Do internal calls use the SIP trunk?

No. Calls between team members stay on the platform and don't go over the trunk, so they don't use the customer's carrier at all. Only external calls, to and from outside numbers, go over the SIP trunk. That's also why an internal call isn't a useful check that the trunk works before cutover.

Who bills for numbers and calls?

The carrier bills the customer directly for numbers and calls. SessionTalk doesn't add a per-minute markup and doesn't bundle minutes. As a partner, you bill your customer for the phone system on your own terms, and the customer's carrier relationship stays separate from that.

Can I change carriers later?

Yes. Because the trunk is the customer's own, you can switch trunks in the admin portal when rates or needs change. Plan the change like the first connection: add the new trunk, route the numbers, check inbound and outbound calls, then have the carrier move the numbers across.

Do users type SIP settings into their apps?

No. Users are invited by email and set up mobile apps with a QR code or desktop apps with a setup link, so nobody has to type SIP settings or receive config files. The apps run on Windows, macOS, iOS and Android, and the same account works across all of them.

Can an MSP connect a different trunk for each customer?

Yes. Each customer has their own SessionPBX project under your partner account, and you manage the SIP trunk, numbers, extensions and call flows for each one separately. One customer's carrier choice doesn't affect another's. The SessionPBX partner programme page explains how to apply.