When the Phone System Fails: How One Client’s Call Flow Breakdown Triggered a Vital Review

Recently, we helped some of our clients resolve an unexpected issue with their phone system due to license changes within the Microsoft ecosystem.

Their Teams Phone – normally smooth and reliable – suddenly lost the ability to make outbound calls. As soon as they called us, we dug into the setup and quickly identified the root cause: a change in Microsoft’s PSTN licensing that affects how resource accounts handle outbound calls.

It’s the kind of behind-the-scenes shift that can catch businesses off guard, especially with the wider push away from traditional PSTN/ISDN lines and towards full cloud telephony.

This incident highlighted a challenge many organisations may face without realising it, and it’s exactly why our Microsoft 365 expertise guides our clients through every challenge – planned or otherwise.

What Caused the Breakdown?

The setup was standard: the main auto-attendant routes calls based on menu options (“Press 1 for Sales, 2 for Support…”), followed by a standard user domestic calling plan (the default for a long time).

That logic is fully supported by Microsoft documentation on auto-attendants and call queues.

However, the problem arose because the firm had relied on standard Calling Plan licences for those resource accounts (auto-attendants and call queues) to make outbound PSTN (Public Switched Telephone Network) calls.

Upon investigation, we found that this failure is due to a licensing change from Microsoft Corporation.

As reported in a recent article, as of 1^st November 2025, resource accounts that make outbound PSTN calls on behalf of auto-attendants or call queues can no longer rely on Calling Plan licences. This was due to the changing from Microsoft moving away from On-Behalf-Of (OBO) calling to Pay-As-You-Go (PAYG).

Why This Matters for Your Business

Customers had previously invested in migrating away from traditional ISDN lines to Teams-based telephony, with the aim to reduce overhead and simplify user management.

They were using auto-attendants and call queues to give a polished, professional front door to their business, so they wouldn’t need a receptionist to manually route every call.

As Microsoft highlights in their blog, this solution is exactly the value-add that Teams supports: auto-attendants route calls, call queues balance agents and waiting callers, and businesses benefit from streamlined call flows.

However, the licensing change means that unless you review and update your telephony licences, the very benefit you built may stop working. This means that callers will hear silence or receive errors rather than reaching someone.

How We Fixed It

At Outbound, our approach with this client followed three key steps:

1. Audit & Assess: We reviewed all the client’s auto-attendants, call queues, resource accounts, and outbound call flows – especially those that route to external numbers or non-Teams users. We identified which workflows used resource accounts with Calling Plan licences for outbound PSTN.
2. Map & Plan Licence Changes: We mapped which resource accounts must be switched to PAYG licences (or an equivalent compliant model) to ensure continuing outbound call support. We also flagged any inbound workflows or internal call routing that might still rely on legacy assumptions – of which, an Azure subscription is needed for overage calling.
3. Configure & Validate: We implemented the licence changes and tested the call flows. For example: auto-attendant → call queue; outbound call now works correctly under the new licence model. We also documented the impact of not acting, so the business’s leadership understood the risk.

Steps You Should Take Now

If your organisation uses Teams Phone with auto-attendants or call queues – particularly when outbound PSTN calling is handled directly through Microsoft (such as transferring to external numbers or using callback features) – you need to act now:

• Identify all resource accounts tied to auto-attendants and call queues that perform or initiate outbound PSTN calls.
• Check the licence type: If you’re using Calling Plan licences on those resource accounts for outbound PSTN, you’ll need to switch from the domestic calling plans to the PAYG model (or another supported licence) before the deadline.
• Test your call flows after the licence transition. Don’t wait for failure; instead, simulate the scenarios (auto-attendant to external, call queue outbound, transfers to external, etc.).
• Document and budget for the licence change. PAYG models can incur variable charges; ensure cost transparency and forecast accordingly.

Why Partnering with Outbound Makes a Difference

At Outbound, our expert IT consultancy helps organisations migrate and optimise telephony via Microsoft Teams, especially when legacy ISDN/PSTN systems are being switched off or repurposed. Our clients benefit from:

• Comprehensive IT support that keeps systems stable, secure, and aligned with your business goals.
• Robust cyber security measures designed to reduce risk, protect data, and strengthen your overall security posture.
• Deep expertise in Microsoft 365, helping your organisation get more from its tools, licences, and collaboration features.
• Proactive monitoring and guidance to prevent issues before they surface and maintain long-term reliability.
• Clear documentation, ongoing reviews, and strategic advice to ensure your technology continues to evolve with your needs.

Book a Meeting with Us Today

This incident shows how easily organisations can be caught out when licensing changes go unnoticed.

Microsoft’s shift from Calling Plans to PAYG for resource accounts handling outbound PSTN calls means businesses need to review their setup, check which accounts are affected, and update licences before service interruptions occur.

If you’d like to review your Teams telephony licence health, call-flow design or migration plan from ISDN/PSTN to Teams Voice, let’s talk today.  

FAQs

1. What is an Auto-Attendant and why is it important?
   An auto-attendant is a dial-in menu that routes incoming external calls via a set of options (“Press 1 for Sales…”). It helps organisations give a professional, consistent front-door experience and route callers efficiently.
2. What is a Call Queue and when should you use it?
   A Call Queue holds incoming calls when the relevant agents are busy. It ensures callers can wait rather than being dropped and distributes the calls using methods like round-robin, serial, or priority.
3. What exactly is changing with Teams PSTN licensing?
   As of 1^st November 2025, Microsoft no longer allows resource accounts that make outbound PSTN calls on behalf of auto-attendants and call queues to use Calling Plan licences. They require Pay-As-You-Go licences instead.
4. Which organisations are most at risk?
   The organisations most at risk are those using auto-attendants and call queues that route calls to external numbers or rely on callback features, those still using resource accounts with Calling Plans for outbound calls, and those that haven’t recently audited or updated their Teams Voice setup.