The phone is timing out when trying to find the callmanager. SRST is depended upon a trigger on the phone. If the phone does not get a response to the callmanager specified in it's TFTP configuration, it will fall to the SRST specified and register. Somewhere you have a tunnel dropping or too much latency between the phone and the cucm cluster. There may be a timeout you can increase in the service parameters, but I do not have it in front of me. Possibly someone can add some more comments
