Solved! Go to Solution.
No they haven't figured it out. Mine hasn't worked right since November 2017 desipite many attempts from tier 2, tier 3, XM escalation specialists, etc. I give up...
You can make and receive calls on XM iPhone with Cellular Data switched to OFF
However for voicemail notifications to work, Cellular Data has to be switched to ON
It looks like XM delivers the Voicemail Notifications (and the voicemail itself?) to iPhone via Cellular Data.