Avant incoming calls go directly to voicemail
This seems to be a common problem with many makes, models, carriers but T-Mobile and Samsung show up pretty often in searches.
I can only use wifi calling in my home.
I got my phone (actually 5 phones) 1 month ago and initially most incoming calls went to vm.
After numerous calls to tmobile I have worked through various steps, get better results but still have issues. A list:
1) lots of phone and router restarts
2) I use comcast, good internet speed I think. 40 to 100 gig download speeds measured by ooklah on phone
2.1) use wifi perferred only
3) replaced comcast combo router with personal cellspot,
4)replaced modem with new moto surfboard
5) use 5 ghz band for phone only
6) call blocking disabled
7) end all apps when I come in house
Some or all of the above helped a little then
8) somehow cellular data got turned off, I didn't do it on purpose, not sure how it happened (maybe someone is working on my ticket but not keeping me in the loop?), things seem better now, only loosing 10% of calls. I tried turning cellular data back on but next 2 test calls didn't ring)
Of course due to the hassle of switching settings back to modes better suited for being where there is cell coverage I usual don't, which means I am paying quite a bit for a smartphone that doesn't do much.
If anyone has found settings, tips, device models etc that actually work reliably please pass them on.