Start researching your cell providers' modem/data offerings.
Using a modem + a cell phone isn't likely necessary; the cell phone will be acting as a modem, providing an RS-232 plug to the controller, not an analog RJ11 jack.
Find out if your cell provider has a data-only mode. Motorola used to have this; you dialled "*DATA" and were connected to their data network so it didn't try to do voice compression on the info coming from your phone.
I've heard good things about Sprint's PCS Vision system; those picturephones are riding on top of a decent digital wireless data system that's optimized for data packets, not voice throughput. I have a customer who uses those phones for wireless Internet access as you do, and claims regular connections at 56,000 kbps.
What you won't hear from A-B is "sure, that'll work with cell phones". There are hundreds of cell models and dozens of suppliers of service and there's no way for Rockwell to investigate and troubleshoot all of them. At the very least be ready to use configuration scripts (maybe the phones use the AT command set !?) on each side to set up the data rate and compression mechanisms so that your PC's serial port and RSLinx get their packets through in the right sequence to the DF1 port on the SLC-5/03.
AT&T also brags about their embedded data offerings, though those may have been ditched when they discontinued the fixed wireless business a few months ago. The wireless phone business moves even faster than consumer PCs!