phone won't ring calls go straight to voice mail

I had the same problem on one of my lines for many months and Tmobile was unable to fix it. Finally I demanded a new phone number and the problem went away. Now another line (my sister) is getting the same problem consistently when I call that number. However if she calls me, I have no trouble calling her back. But that's good for a few hours at most and then once again my call goes straight to her voicemail. Now I notice I'm starting to get missed calls on my own line (main), no ring and no log but only a voicemail. Came close to having a real estate deal messed up because of that.