My wife and I have also had this problem since switching from T-Mobile. We tried to bring our 6S+ and 6S, but only the 6S+ worked with XM. So I have a 7 through Xfinity, but both are having the dropped calls.
I called Xfinity a few days ago and the rep said they are working on a network fix/update here in California (Bay Area) that should be done by midnight Tuesday. I complained again today as my wife has phone interviews for jobs that she needs the phone to be working in good order. The lady tried doing an analytics check, but the popup on my phone never showed up to allow them analytics access. She said I have to go into a store to check it. She also said that after Tuesday, to do a Erase Network Settings and then power off the phone for 5-10 minutes and it should work.
I'm currently looking into Verizon prices...
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