There are only a couple reasons for IP phones to reboot. The one we see most is missed keepalives to or from the callmanager. Get a detailed callmanager trace from the time when one of the phones resets, go through the trace and find where the phone reboots. Go up from there and look for where the keppalives stopped. Also clear the interface counters on both routers and check them after a phone reboots..any errors, drops, resets?
