How do you get online? A lot of businesses are starting to transition to computerized processing through their DSL lines. I assume if you don't have a landline, you must have cable or satellite internet, why not use what you're already paying for to process credit cards? Just install a POS on your computer that supports your equipment, which more than likely has a serial or lan connection.
I don't think it's such a good idea to use your cell phone for processing cards... but I'm sure it's possible if your phone is capable of being used as a modem.
