phone won't ring calls go straight to voice mail

I am having the same problem! Only certain people were able to call through. I missed so many calls! I had random voicemails with no missed calls. Call forwarding is not on, I have service. Not sure what the issue is but this is the first time in about a year that I had T-Mobile that I had this issue. I'm quite annoyed.

and I have an iPhone. You'd think that would matter even FaceTime don't work sometimes!