1 - If the iPhone is LOCKED to some carrier* just put a SIM card that specific carrier.
2 - If the iPhone is FACTORY UNLOCKED* just put a SIM card from ANY carrier.
If you dont get any service with the SIM card inserted on either the scenarios* your iPhone is blacklisted.
Nevertheless* if it does work and if the iPhone came overseas (from your accent* I take you like in the UK)* then its possible it still might be blacklisted in some other country.
One last thing* go to SETTINGS > GENERAL > ABOUT and tell us what you see in the NETWORK* CARRIER and MODEL specifications.