off-topic apologies - how to unlock sim (not phone)

I apologize for this in advance. I figure the collective GSM knowledge here might be able to help me. I have a SIM card that can only be used on one phone. The phone itself is unlocked. But the SIM won't work on any other phone. Does anyone know anything about this or how to unlock a SIM to work on any phone? Yes, I have tried searching, but because PHONE unlocking is such a common theme, this just passes through the cracks. Thanks, Scott