I need a SOHO router that will fall back to dialup when dsl / cable fail. I learned from that Web that the RF550, RF560vpn and RF820 has that option, which model should I choose?

You should go with the new RF820. RF550 / 560 are old legacy products.

The RF820 has better firewall protection and better control of assigning access to the LAN users. The RF820 also has better hardware and software in terms of overall performance.