I want to use my cellphone on a different network?

I have several cell[hones that I carry with me. I just got a new phone from one of my providers, and I am not fond of it. I have a bunch of nearly new out of service cellphones in my drawer. I realy like one of them, and I want to switch it.

Of course, the problem is, it is locked to a different network. I need to unlock it, so it will accept a SIM card from another service provider..

The service provider refuses to unlock it. I pointed out that it is my phone, I paid for it, but of course, they refuse. They just stand there and look at me like I am the problem.

So, there are all kinds of unlocking sites, almost all listed as free. Trouble is, when you ask them, they want your credit card so they can get money. (usually $39.95)(U.S.) So, they are NOT free.

Some of them don't ask for the IMEI #, which they need to find the code. I don't trust them. I have given my credit card # a few times for sites that look legit, and got nothing but a charge on my card.

I have an unlock #, but I am afraid to use it, because it really was free.

Any suggestions on how to handle this will be appreciated. The phone I want to use is a Samsung Galaxy 551