Can an installment plan be paid for on a separate bill?

An installment plan is tied to a line. If your line is eligible, you can purchase a new phone on an installment plan using your line and give it to him. There is no way for you to have an installment on your bill for a line not on your bill. 

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@ksmotherman wrote:

Is it possible to pay for a new phone for someone on an installment plan on my bill, if they are on their own data plan already?

 My spouse is still bundled on a family plan and I am on a separate plan. Wondering how difficult it would be to be able to “upgrade him” and I pay the installments on my plan until he is able to move from the bundled plan onto mine later? Obviously would like to get him a new phone without his knowledge.

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The only way with AT&T would be is if you don't owe for a new phone, you could buy it on your line and give it to him IF he's on AT&T (it would work on the other account).

The other way would be to buy it somewhere else, for example: if it's an iPhone you can buy it on no-interest payments from Apple.

BUT, if his family plan is with someone else you don't want to do that, you want him to join when they are doing a promo for a free or discounted phone (which often requires a new line).