What does it mean when a cell phone keeps going to busy signal?

A lot of people who tried to reach me told me when calling my phone it makes a busy sound. I tried calling my phone and it’d sometimes ring a few times then go to voice mail, go to voice mail, ring normally then voice mail, but MOST times just go to “All circuits are busy” like 90% of the time. I don’t think anythings wrong with my phone cuz I used it last night. I have the Droid HTC HD2 with Tmobile.

What could be wrong? It’s not time to pay the bills yet.