Looking for info to hook up my 836i to a credit card terminal

We do craft shows and at present, are using a verizon bagphone with a model & a battery & a credit card terminal for our credit card transactions. I was told recently that Verizon at the end of December will no longer offer analog and that we will have to find another method. I have a Nextel 836i cellphone & was wondering if anybody knew where I could get a connection cable to hook up the cellphone to the modem. Thanks in advance for your help.