This means that as soon as you PC is swtched off Phone loses communication with CUCM but it is still able to communicate with local SRST router.
Can you provide the following info -
1. Does this happen with all the phones or just one phone?
2. Is this specific to phone model? If so, what is the phone model?
3. What switch is the phone connected to?
4. What is the phone load version?
-=-
Udit