Symptom:
This behavior is due to speed mismatch which happens when the laptop  goes into hibernate or sleep mode, when the laptop goes into this state it NIC interface reduce the interface speed to 10b/s since we have auto negotiate feature on the phone. The phone would make sure that same speed is been matched on the other side of connection which is in between the switch and phone.
IP phone have a limitation where they can't keep themselves register at such a low speed and lose the connection. However once you unplug the  LAN Cable from the Laptop which is connected via the IP Phone they will re-register.

Conditions:
On Cisco CallManager 
CCMAdmin -> Device -> Phone
Item:
Auto Port Negotiation? enable on the Phone