1. You need to find either the receipt for the sim card or the packaging, both of which will have the key # printed on them.
If you bought the sim from someone and they've not handed these over, then you've been diddled.
2. I don't understand why you rang two companies for help. Is it because the handset is locked to one company and the sim is for the other?
3. Find out for certain whether the handset is 'unlocked'/'jailbroken' or still locked to a provider and, if so, which one. Ring that company, tell them the IMEI number and say you are about to buy this phone and want to check to see if it is reported lost/stolen.
4. If all is okay, you can ring again to say you bought it, the next day snd then go through the steps to transfer the IMEI to your name and address and unlocking it to allow your sim to work in it.
Hope that helps.