Don't you have caller-ID block in your area? In my area, I pick up the phone, dial *67, and then after the funny-sounding dial tone, I simply finish dialing the number I wish to call.
It shows up on their end as something like "Unknown number" or "Private call/number"....something like that.
I'd call their toll-free number from your house, since it costs them when you call, rather than spend a dime trying to figure out why the heck the guy is calling you. Before giving them ANY information, I'd first find out what the company is, what their mailing address is, and who you are speaking to. After getting all of that information, THEN start tracking down who/what/why you keep getting calls. If need be, go to OfficeMax/OfficeDepot and see if they have some pre-setup documents for stuff like this (oddly enough, they have stuff like divorce papers, rental agreements, wills, etc). Send them a nice kiss my arse letter to the address you were given.