Trying to make a phone call disconnects immediately

[KingAlfredo]

I have a one plus 9. I had this problem before I swapped APN's to the newest one and I'm still having problems. When I try to call or even access the voice mail it immediately cuts out without a dial tone 

[softech]

@KingAlfredo it's a VoLTE issue.  The phone was trying to use VoLTE but it is not working well

do you see a VoLTE toggle at the Connection screen? likely Settings> Wifi and Internet > Sim and Network > Sim 1 (or Sim 2)

If you see that, turn it off, then you can change it back to 4G or Automatic.  With VoLTE turned off, the data will use 4G and when calls come, it will go back to 3G for the calls

[KingAlfredo]

Interesting. Yes 3g makes the calling work

[softech]

 @KingAlfredo 

APN only has to do with data and not voice call

The voice call problem could either be a VoLTE problem or 3G problem.  Can you try changing the network mode to 3G Only and test calls again?

also, check your connection screen and see if VoLTE toggle is there, if so, disable it. 

(Settings> Wifi and Internet > Sim and Network > Sim 1. You should see a section for Enhanced communications to toggle VoLTE)