I have a solution that I use it is cheap and I use it anyways, I set up an account with paymentech they were mentioned above, I use Their Touch Tone Verification POS, and I use the normal zen cart module for credit card, it's not real time, but it works and it is pretty affordable.

Advantage for me is I do a lot of conference and trade shows where I also sell my product so I can bring my cell call the 800 number and verify credit cards right there, I don't need another piece of equipment to bring and I don't need to pay any extra fees for a phone line at shows, so it has been an all around way of doing things. Not real time, but I don't ship that very second anyways so it is negligible.