Why is my iPhone 13 not ringing for certain incoming calls and dropping outgoing calls?

My iPhone 13 has recently stopped ringing on certain incoming calls from my contacts. I can be sitting right with my phone and can see the call coming in but there is no ring and the ringer hasn't been turned off. On out coming call, frequently after dialing the phone will drop the call before it even rings. I don't know if these problems are OSI systems related or an internet provider issue?

[Re-Titled by Moderator]