Some users not receiving Voice Calls

I am running into an issue of some users not receiving voice calls but other users do receive them. Has anyone had this issue? Even when they are in the same On-call roster, only some users receive the calls.


Have you tried testing with having one of the issue user alone in a roster? Will it call out to them then?

Is there any commonality between all of the issue users? Same phone carriers (Verizon, AT&T, etc)?