Incoming Call Queue Calls - Transfer Failed

We use teams calling (Co-Existence set to Teams Only) for our calling center, have not had any issues until about 4-5 months ago when we lost the ability to transfer calls when customers dial in to our call queue. This is the call flow:

End users call into our phone number (either the toll free or local number) > greeted with the call queue/available options > user selects applicable options and is routed to the applicable team. A Technician answers the call via the desktop client. If the technician needs to transfer to an external number (IE an employees cell phone), the call fails and the end user is hung up on. At this point I've escalated and escalated internally with Microsoft to no avail. After troubleshooting we noticed that through the mobile application, using the exact same call in process, we can transfer to external phone numbers; this is the same case for the web app. Both of these options are less than ideal. I am just curious if anyone has witnessed these same issues, and what was done to resolve them? Microsoft support has not been able to help at all and our ticket is 2-3 months stale at this point with no progress besides a "we are still working on this but have no idea" daily email.