Carier network for iphone 4?

Carier network for iphone 4?

If it is a used phone it will likely have the carriers brand name on it. If it is unlocked with a SIM Card T-mobile, AT&T, and T-mobile are options.
You might be able to see the Carrier on a used phone when you turn it on because it will likely log into it even if only for 911 call of unactivated phones