keeps on calling and calling and never anyone on the other end

i keep getting calls from this number . It started around Nov 27th and has continued daily...... Several times a day as a matter of fact. It's always the same last 4 digits on my cell ID. This makes me believe it's not a company or certaintly not a large one if they only have 1 line. I still haven't answered the phone when they call and i'm not planning to. 302 is the area code for Delaware. Where the hell is Delaware anyway??

They call using a fax machine and squeal in my ear. This hurts, plus it's a business taking away time from my work.

I received a call from this number at 11:55 p.m. I did not answer but looked up the number this morning to see if I could figure out who was calling at that time.

I have no idea who this is. I tried to call the number back and it says Portfolio Recovery and to hold. There is no reason they should be calling me and I wish they would stop.