Jailbreak your iPhone and install Terminal, OpenSSH server and Minicom. Change the root password using the passwd command (assuming your iOS is a version in which passwd works) before enabling the SSH server. The default password is alpine, [[Artnid:325475|and it's commonly known]], so you really need to change it.
Once installed, open up Terminal or SSH in, start Minicom and change the settings - same as you would on a linux box. The device is /dev/tty.iap, speed 9600 8N1, no hardware or software flow control. Remove all the device initialisation, dialling and hangup strings, and save the new configuration as something, preferrably Cisco. Save and exit.
Restart Minicom with your new config by typing 'minicom cisco' and you're done. SSH into it from a windows machine on-site or via an iPad and you're set. 3G iPhones with smashed screens are cheap, you can just leave one in your bag with the serial port, and you never have to worry about being locked out of a device.