M2M and M2AM are not available for AT&T PrepaidSM.
About Mobile to Mobile calls
With Mobile to Mobile calling you can make calls to or receive calls from other wireless numbers without using your Anytime minutes. To qualify, you must be within your rate’s calling area in the U.S., Puerto Rico, or U.S. Virgin Islands.
There are two types of Mobile to Mobile calling:
Important to know: International long distance calls and International roaming calls are not included. Calls to directory assistance, voicemail, and calls made to/received from mobile toll-free numbers, mobile chat lines, calling applications, numbers for call routing and call forwarding services, and machine-to-machine numbers are not included. Only numbers in the industry wireless number database that AT&T uses will be treated as a call to/from a mobile number. Numbers belonging to other carriers which are not in the industry wireless database and numbers not reported as wireless numbers are not included. Coverage is not available everywhere.
Use the AT&T Mobile to Mobile Lookup Tool
From a desktop or tablet, follow these steps to use the Mobile to Mobile Lookup Tool to verify if a wireless number qualifies.
How to add Messaging Unlimited with Mobile to Any Mobile Calling
Go to My Wireless Services to add the following features to your existing qualified AT&T Nation and FamilyTalk plan:
Ineligible voice plans
Messaging Unlimited with Mobile to Any Mobile Calling features are not available for the following rate plans: