Called saying I owed them money for a loan from Cash Advance

I work in the telecom industry and this appears to be an operator service company that provides service to hotels, hospitals, prisons and payphones. The reason it shows up as a dallas number is that is where the operator center resides but calls could come from any where in the states. I had a couple calls from my cousin that I did not accept but my guess it was from a jail or prison.

They constantly call and do not answer, and do not leave a voicemail. I have not picked up for the past few days, but they still insist on calling constantly. I've tried calling back, but it hangs up before it even rings.

Called back aand it says the number is unallocated

this number calls and says nothing and hangs up when i answer it.