Skype for Business audio call to Teams does not notify Teams user of an inbound call
User A is using Teams
User B is using Skype for Business and has A in their SfB contact list.
B initiates a call to A as a Skype audio call.
User A hears an audio indicator via headset or speaker that an inbound call is ringing but no on-screen popup occurs to offer a way to answer or decline the call.
Both users are licensed with Enterprise Voice. If B calls A using the PSTN number assigned to A, no problem.
Issues occur when B uses the SfB default to simply make a non-PSTN audio call between the two users.
Fix -- properly display the toast notification that an inbound call is present.
I was able to correct this recently by disabling SfB for inbound calling. There was a process to turn off hybrid mode. I think this is a temporary state until all functionality is moved to teams and you can turn off SfB for all users.