Don't come to Libya expecting to make a roaming call on your non-Libya service provider's SIM card. Very few GSM operators have roaming agreements with the Libya GSM operators so you'll need a local SIM card.
There are two main providers, Libyana and Al Madar, of which Al Madar seems to have the better network coverage.
To get a SIM card, you'll have to go to their head office in Tripoli with a copy of your passport and 10 Lyd and fill in the forms....These people like their fcuking paperwork!
Getting hold of recharge scratch cards isn't a problem, most smaller grocery stores and supermarkets sell them in 10, 20 and 40 Lyd denominations.
Local calls are cheap, for international calls you'll need to load up first coz your credit will go down like a V8 petrol tank with your boot on the loud pedal...