AT&T customers who live or work in one of the company’s many pockets of poor or dead 3G service will soon have the option to pay $20 extra to obtain unlimited calling over 3G using their own Internet access.

AT&T hasn’t yet set a retail price on its forthcoming new 3G MicroCell appliance, which connects to your existing Internet to supply local 3G voice, SMS/MMS, and data coverage for nearby AT&T customers. However, a report by Engadget Mobile attributed to an anonymous tipster says that the company plans to offer an unlimited calling plan for users for $20 per month.

Bundled with AT&T-supplied Internet (if available in your area), the monthly fee drops to $10, and with AT&T Internet and landline service (if you also pay to have one of those old plug-in type phones), the monthly fee goes away entirely.

It’s not clear if the monthly fee for unlimited wireless calling through the device will be mandatory in order to obtain the 3G MicroCell device. According to the photo of official looking marketing collateral included in the report, “3G phones connected to the MicroCell without AT&T Unlimited MicroCell Calling continue to use existing plan minutes.”

If unlimited calling is entirely optional, iPhone 3G and 3GS users may be able to buy and install the 3G MicroCell and simply use it to burn their existing plan minutes without paying any additional monthly fees, solving dropped call or delayed SMS issues for users working inside shielded buildings, for example.