Stop calling me!!!

I constantly receive phone calls from somebody, asking for my husband and as soon as I ask who is calling or that he's not there, they hang up on me. I've tried to call the number back, but I only hear the busy signal. On my Caller ID it tells me that there was no name found, but the number is 11-918-442-0303. I've also tried to find out online where this call came from,... but no result! We've signed up on the Do-Not-Call-List on www.donotcall.gov,... but still receive these unsolicited calls!