The Cisco 7910 is an SCCP or "Skinny" protocol only phone. You can read up on the general how-to's with Asterisk here:
VoIP-Info Cisco Phones You will need to read the part about configuring chan_skinny in order to get the phone to work.
I have tried to use Cisco phones before (7940/7960) and had horrible problems getting them to stay connected. It seems that about once every week or so they would bomb out requiring them to be reset. I am sure there may have been some kind of misconfiguration on my part but after 100 hours of going over them I gave up.
As a side note, I would highly recommend a phone that is meant to work outside of the manufacturer's sand box. More specifically a Polycom or Snom phone. Something like a Snom 300 or Polycom IP 320/330 would be a more then adequate replacement and also a mild upgrade from the Cisco. Both of these manufacturers specifically do interoperability testing with Asterisk to make sure that features will work as advertised. Your mileage may vary.
