If your Microsoft Teams Strategy is to embrace the native tools (and costs) Microsoft allows the information below outlines your options. The advantages of using the Microsoft Calls application is the Teams user has the most native integrated experience in Teams. Specifically, with the TeamMate Connector you have the option to add a custom built, and branded, Teams Application as well – this is covered in Choice 3 below.
Going Native means you have purchased the Phone System or Microsoft Business Voice license for your O365 user(s) or you are already using a User license where this is included – E5, A5, etc.
Here are your choices from there:
Choice 1: Use Microsoft Teams for Calls application, PBX features, calling, and phone numbers. This is the nuclear option – move everything to Microsoft Teams so you get one bill from one provider. Features are not mature, support is difficult and customization is almost impossible.
Choice 2: Use Microsoft Teams for Calls application and PBX features only. This is when you create a Direct Route connection from Microsoft Teams to any SIP Trunk service provider. You pay Microsoft for PBX features and a Service Provider for calling and phone numbers.
Choice 3: Use Microsoft Teams for the Calls application only. This is where you create a Direct Route connection and register the Microsoft Teams user on their existing PBX. Microsoft Teams Calls becomes a PBX softphone. With TeamMate, you can also build a custom Microsoft Teams PBX Application that provides voicemail notifications, PBX softkeys, and the PBX end-user portal (if you have one) all in Teams.