Does anyone need to migrate from a legacy premises-based phone system to MS Teams and utilize it as a full-featured phone system, taking advantage of the free phone system included in the E5/M5 license pack?
What challenges (if any) have others faced when attempting to use MS Teams as a full legacy PBX phone system replacement? Our clients' challenges were getting the legacy services such as analog, faxing, agent log-in/out, true inter-office enterprise extension dialing, etc. to work.
Yes to use the Microsoft Phone System in Microsoft Teams you need to migrate somehow. Microsoft offers Calling Plans for some countries so you port your current phone numbers to Microsoft. If you are not located in one of the countries that Microsoft supports with Calling Plans you can install a SBC On-Premises and connect any SIP trunk from your SIP trunk provider, this SBC will connect you to Teams with Direct Routing. Some telco providers also provide a partner hosted SBC and then you don't need to host a SBC.
I would not say that it is free since you still have to pay for your licenses and in some cases you will need a SBC or an agreement with your telco provider, but the function is included in E5 or you can buy the Phone System Add-On license.