If you have a Cisco 7962G with the latest (from Cisco) 8.5(4) SIP Firmware, and it works very well for the most part, but after less then a week of heavy usage, eventually the phone gets into a state where it won’t accept or let you place any more calls, the screen flashes “no free lines available” or something along those lines. (power cycle fixes this). If you have tried to upgrade to the v9.0(3) firmware, you’ll notice that when that’s loaded, the phone won’t register with Asterisk anymore.
It would be advisable to switch to Polycom, but if you really must use Cisco phones, downgrade to 7.5. People have got a lot of 79xx phones out there and 7.5 is the last stable release as far as I’m concerned. It just seems to work, no periodic reboots needed, or any other quirkiness like with the newer firmware’s. The feature set is not robust, but it is reliable with Asterisk. Keep in mind, Cisco has no incentive to make their SIP firmware work with any other platform other than there servers so I don’t really expect it to work properly if at all with Asterisk. 7.5 is the only firmware version that just works fine.