disconnected number keeps calling me

Instead of ignoring it, answer it the next time they call and tell them they have the wrong number.

I believe it is an robo dialer which makes the call. Is it somehow related to collections? The best idea would be to pick up the phone and find out. A similar number is used by UGDI Health telemarketers.

This is a very common scenario. It's very easy to have a caller I.D. to a number that doesn't even exist, yet the call could come from a real live working call-out number. This doesn't surprise me. However, more often than not there usually is some recording on the other end.

It's probably a spoofed number. Scammers hide their real phone numbers by faking the information that appears on the recipient's caller ID. This explains why some calls come from disconnected numbers, weird 4-digit numbers or even alphanumeric codes instead of a phone number.
But be aware that although the information on your caller ID is fake, the call is certainly recorded within the telephone company's database. You can have the call pulled up based on the time and date you received it. The telephone company can give you at least the caller's phone number. You can have him blocked, too.

I got a call like that before, too. But mine came from a caller which registered an alphanumeric code on my caller ID, instead of a phone number. I decided to look up the possible reason online and I found the answer here: http://www.callercenter.com/articles/calleridspoofing.html. The phone number was indeed spoofed. But I still found out who the caller was by referring the call to my telephone service provider. ;);)