Dropped Calls for Long Distance Calls on iPhone 12

Hi:  I am having an awful time with calls dropping every 2-3 minutes.  It seems to be just the long-distance calls and it's almost like I'm on a timer.  This all of the sudden just started happening back in November.  I thought a new phone would solve the issue, but now I'm on my second new phone and still dropping calls.  Of course, I called support and they simply cannot help me.  Is it time to switch to a new provider?  Any suggestions?