This is a common ploy by criminals masquerading as debt collectors who are attempting to extort money from people by scaring you into believing that you will be criminally charged, go to jail, lose your driver’s license, have wages garnished, be sued, and a variety of other variations on this,  all for an alleged or nonexistent debt.  One of the tricks they use is to call your work place, friends, or relatives repeatedly in an attempt to shame you into paying.  They also use the “process server” ruse who calls and claims he is going to serve you, but then says you could avoid it by calling another number

Federal law (FDCPA) requires them to send you a letter (US MAIL ONLY) within 5 days of their first contact that contains their name, physical address, the creditor’s name, and the amount of the alleged debt.  It also must contains “mini-Miranda” telling you that it is an attempt to collect a debt and that all information will be used for those purposes.  The one other important thing that this letter must also have in it is that you have a right to dispute the debt within 30 days of receipt of the letter and if you do so, all collection activity must be stopped until the debt is verified.
Read up on your rights here, get template letters to send and also make a complaint at this government site: http://www.consumerfinance.gov/

Also file a complaint with your State Attorney General's office.
List of State AG’s offices:  http://consumerfraudreporting.org/stateattorneygenerallist.php

Was a wrong number and they left a voicemail.  It's an automated dialing service.  They are a Service of Process company hired to track down and serve papers for lawsuits.

Not a debt collector.  They are trying to serve you papers for a lawsuit.  They were hired by someone else who is trying to collect from you.  They wont provide you proof because they are not the loan company, they are just contractors helping the loan company sue you.  Check your credit report.

What a load of bull excrement. If they're trying to collect they are legally required to provide the documentation: Both documentation of the debt itself and of their authorization to collect it.

by refusing to provide documentation these "debt collectors" are inviolation of the law and should be reported to the authorities as quickly as possible. See the links Big A provided.

Well we can see that you are another "reading is fundamental" failure.  The poster you are making that idiotic remark to never said anything about a process server, but since you seem to know what this scam is about, you obviously are in on it.  Now try to reading my post right above yours and try to comprehend it.  If you can't comprehend it perhaps you can get something with more intelligence than you to explain it to you, probably a gnat would have a big enough brain to understand it.  Your shilling attempt has failed.