Since 7905 is only supported as a skinny phone starting CCM 3.3.2, the only option you right now is to configure the 7905 in H323 mode, and then set it up as a h323 client in your 3.1(3a) CCM.
You can get the 7905 H323 load in the following link, the zip file includes a few utility which you need to set the phone up.
http://www/cgi-bin/tablebuild.pl/ip-phone-7905
Here are a few additional links for configuring the 7905 in H323 mode.
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_ipphon/7905_ip/7905admn/7905h323/7905ach4.htm
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_ipphon/7905_ip/7905admn/7905h323/7905ach3.htm#wp1002608
-ben
