phone won't ring calls go straight to voice mail

I’ve been plagued by the same problem for months since I switched to T-Mobile.    Customer service has been totally not helpful.  When it happens I’ve received a text that the call has gone directly to voice mail from my contact.  They try again and this time it goes thru.  It’s very inconsistant.   Has to be a T-Mobile tower issue.  Not enough bandwith for all the activity.  Strange that this has been going on for years with no resolution.  I will be leaving T-Mobile shortly.  Cheapest is not always the best!