Who is 'they' that is asking for your SIM info? If you are talking about a service provider/carrier, like you are trying to change from one carrier to another, then if your phone doesn't use a SIM and they use SIMs, you can't use that service.
There are two kinds of networks cell companies use CDMA and GSM. You will need to know which one your phone is before you can change to another provider.
Say you had Sprint or Verizon, and you were trying to change to AT&T, you can't. Not using that same phone. The first two are CDMA networks and the latter is a GSM network. They are not interchangeable and you cannot convert a phone from one to the other.
Now if you are talking about something else, I would need more details.