You could have 15 ATAs each with two lines on the telemarketing plan ($149/mo, I think). Problem is that you would need a considerable amount of upstream bandwidth (90 kbps x 30 lines = 2700 kbps) to use each simultaneously.
If you really want to do fax blasting I think commercial services would be a much better deal. Search google for fax broadcasting and you'll have plenty of choices.
