If you have a GSM compatible mobile you will be able to use your foreign handset in Portugal. To avoid running up huge bills, it is a good idea to buy a Portuguese SIM card.
It is possible that you will need your phone unlocking in order to use a Portuguese SIM card. In Portugal it is extremely easy to find a place that unlocks cellphones.
There are three major mobile phone service providers in Portugal, Vodafone , Optimus , and TMN . Each provider offers a variety of prepaid (Pré-Pagos) and contract (pós-pagos) SIM cards, both of which are available to foreigners.
To obtain a prepaid SIM card you should go to one of the network provider's shops. There you will be asked for your name and address. No proof of address is required but they may wish to photocopy your passport or ID card.
You can top up your credit with a minimum payment of €5 by:
Getting a contract SIM card is easy, but you will need a Portuguese tax number to apply for a mobile contract. You will also need to bring along your passport or ID card and a Proof of address (bank statement or utility bill).
Payments for contracts can be made in the same way as for prepaid phones. However, there is the option of setting-up a Direct Debit.