If you have a family plan with Metro PCS, you cannot just pay one bill and not the other.  Unless, you call customer service and you separate the lines into their own account.  Keep in mind that you will lose any family plan discount when you separate lines that are on the account. Metro PCS offers a $5 discount for each line on the basic family plan.  When you separate a line off a family plan, usually the line being separated would have to be paid right away to avoid any service interruption.