Kai, the 7100 is out of software support and needs to be upgraded to 11.4.5 for troubleshooting.
The Soundpoint 601 has been discontinued by Polycom and they no longer provide software support for it. We need to verify that the 7100 is not sending any messaging during the call that cause the reboot and that POE is not going out, but if neither of those is the case, we wouldn't be able to open a ticket with Polycom for that phone.
Thanks
Jay