I went from ATT to TMobile. No problem with ATT app for voicemail but I didn't disable it when went to TM. So when I powered up in the morning, it would tell me that voicemail was not available on this phone.
An ATT rep disabled and then all calls went to TM's Visual VM app which I didn't have in the first place.
Except........if wifi gets weak, it turns itself off. How annoying! Gotta go back in and re-connect it everytime.
Is that happening to you?