Putting a sim card in a unlocked phone worries?

Hi, I recently broke my android device in the rain which was not covered by the warranty so instead of buying a new phone all together ive been thinking f getting a used iPhone 3g. This phone is from a different provider but is unlocked. Once i put my sim card in will i maintain all of the features I am currently paying for on my contract? (Unlimited internet,facebook,youtube, ect)

I only ask because I put my sim card in a old flip phone (got from company while mine was out for repairs) a few months back and did not get access to the internet.

I am not sure, You need to contact the customer support of your network as why the internet did not work in your old flip phone.Maybe it had a lower frequency (2G) and you were on a 3G....I do not know the exact reason.But if you use your sim in iPhone 3G you will not face any problems,since its unlocked and a higher version mobile model.Recently,I unlocked my iPhone 3G using the support of Onlinegsmunlock.com and till now using my contract sim without any problem.