How do I send text messages using AIM?
To send a text message using AIM for iPhone, tap the IMs button and then tap the new IM button in the upper right corner of the screen. Enter your buddy's full phone number like: +14085551212. You can also enter just the 10 digit number and we'll add the +1 for you. You can also add phone numbers to your Buddy List view by entering the full number in the Add Buddy panel. Currently, you can only send messages to mobile phones in North America (phone numbers in country code 1).

Can my buddy reply to a text message I sent using AIM?
In most cases, yes, but it depends on your buddy's mobile phone, carrier, and calling plan. Your buddy may be charged standard text message rates to reply.
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AIM on iPhone: Frequently Asked Questions - Discover AOL