Russ,
2 solutions:
One Call Equinox and see if they have their Legacy Products:
http://www.equinox.com/product/legacy.htm
The bottom one sounds like it meets your criteria.
Two. If they have modems and not serial ports,
then you have to get an analog modem pool. The use a dialscript to
issue a dial. One the modem pool modems set them to auto answer.. etc.. I don't have the exact strings on hand, but can get them if you need them. Just connect phone lines between the modems, they will answer even without going through the telco switch first.
Port Master won't work in this case if you only have modem outs, unless you dangle modems off each of the serial ports of the PM2. If you have serial.
If you do have serial outs, then any term server that supports SLIP, CSLIP, PPP will work. Just depending on what stack you have on your POS (no no... Point of Sale.. though I guess it could be the other POS too) device