Solved. Your multicast remarks triggered me, I checked the router for more multicast related settings and found that the router also had multicast flood control which kicked in and killed the multicast. Resetting the phones via multicast didn't generate enough data to trigger the floodcontrol. The actual paging did trigger the floodcontrol.
I switched the floodcontrol of and everything works like a charm.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...