Only if the primary account holder gave you the authority on the account but your phone will be locked to T-Mobile (if it is a branded TMO phone) and unless it is paid off, account in good standing and the primary account holder requests an unlock it will not be unlocked and useless on another network.
With that being said unless you have the permissions set by the primary account holder to make account decisions they (the PAH) own the line and number.
Doubtful your plan would transfer to another carrier. Most have different prices and plan details.
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