I was having a similar issue with DTMF tones not being recognized by my community's call box in order to open the gate. Whenever someone called to get in (lawn guy, delivery, etc) I couldn't get the gate to open for them. After trying everything I had read and different combinations of "fixes" I had found on a lot of other posts, I seem to have come across a solution that worked for my situation.
**My network is AT&T so this may not apply to everyone with a similar issue.
- Settings>Cellular>Enable LTE>Data Only
- Turn off Wi-Fi
- Turn off Bluetooth
- Turn off Personal Hotspot
This means I'll either have to run with all these things off all the time or have people call me twice so I can make the systemic changes between calls and let them in on the second call. It's not ideal but it seems to work (so far).
Hope this helps anyone with a similar issue that isn't getting anywhere with tech support.