you can reboot single sip phones through the monitor application , but i would run some traces check you are getting correct messaging with only the sip filters ticked , call between sip device and sip device and then back again and see the real time log the monitor trace to file .
then google sip messages and by getting a sip message / response list and comparing it with your monitor trace (not under the status tab that will give you a real view time view and hopefully clue and will let you reboot the phone individually)look at what the sip trace log says it looks complicated at first but TX at the start of say a 100 messsage is what the phone is sending and then RX is what it is getting back from your far end.
you usually get x2 bad responses first then from there follow the trace it does become easier it will probably be authentication , if you struggle post and hopefully , I can give you some debugging clues. And between us solve the proble , plus post youre config
APSS (SME)
ACSS (SME)