your question could be answered soooo easily just by looking up the word 'unlocked' in a search, either our site's search or google.

"unlocked phone" by definition means the subsidy lock that restricts the phone to one carrier has been removed. that means an unlocked GSM phone will work with any GSM SIM card as long as the phone supports the same frequency bands that your carrier uses in your area, Cingular and most GSM providers use either 850mhz or 1900mhz (or both) in the US and Canada.

i have an unlocked gsm phone, is it possble to switch it with my existing cingular service that is with another phone, just by removing the sim from the old phone and putting it into the unlocked phone?

THANKS