I guess this issue happens to many communication apps as it happens to me with the Cisco Jabber app.
See if you can reproduce the issue with Guided Access. If so, then it's the app owner that needs to resolve this issue.
Check out my blog post below and see if it helps.
Thank you, I performed that test and it had the same issue so we need to escalate with Skype/Microsoft.
Interesting. We didn't experience any issue with Cisco Jabber picking up incoming calls when Guided Access is enabled instead of Single App mode. Cisco also confirmed it's an issue on their end so we are waiting for a fix now either from Cisco or Apple.
Would that indicate it's a Skype for Business issue then? The call is visible at the top of the screen but with no way of answering.
While both apps share similarities, it could easily be an MS or Apple issue. I would start with MS first.