Can I finance a phone on an existing line?

If you are eligible, you can by using NEXT, which allows you to pay for the phone over 24 months.

Depending on the phone you are looking at, you can buy some through the manufacturer and they will allow financing. Apple for one has financing through their site.

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@Sansarx wrote:
If I have an existing line but no phone for it can I finance a new phone so I don't have to pay the full price for the smartphone?

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I'm not exactly sure what you're asking, but it sounds like you want a discount on purchsing the phone, and you want AT&T to finance it for you too.  I don't think you can have both.  You can get free financing using the AT&T NEXT program, but you pay the full price for the phone.  You can get a contract subsidized discount on the purchase of a smartphone, but you have to pay your portion of the purchase price up front (no financing there).  AT&T does not offer financing for contract subsidized phones at this time, so if you are looking at a 16GB iPhone 6, then you will have to pay $199 up gront to get the phone on a 2 year service contract, or finance the full $649 using AT&T NEXT.

In addition to what has been said, you'll also have to add a data plan to the line if it doesn't already have one.