Does iPhone IMEI remains blacklisted when reported stolen, even if connected?

My sister lost her iPhone. Mom reported it stolen, so they suspended the service (which didn't help me trying to use find my iPhone)… We reconnected it. If the thief were to take out the SIM card, would the phone still be blocked from reactivating it at t-mobile?

The phone can be unblocked and still can be used with some other carrier.