Hi. A customer has a rather thick wall at their site. Itsmaking mincemeat of the signal to their 7921 IP phone. Can I attatch different antennae's to the 800 series router? specifically something omni directional?
You probably want something other than omnidirectional, maybe a patch antenna so you can blast through the wall.
Yes, you can replace antennas. You should be able to unscrew the rubber duck antennas and connect a different one. Cisco uses an RP-TNC connectors for their antenna types.
Here's a listing of Cisco antennas. Make sure to get a 2.4GHz one.