I'm running a 4621SW IP phone (FW 2_6)-CLAN in our G3SI.
I've had it working since July 2006.
System: G3siV12 Software Version: R012i.02.1.118.1
I know you asked about an S8700, but wanted to input my setup to see if that helps.
I read the "vpnphone_readme.htm" and
Section II: System-Requirements
1. IP Telephone models 4610, 4620, 4621, 4622 and 4625
2. Security Gateway Firmware version 4.6.22 or above
3. Cisco VPN 3000 Series Concentrators
4. Cisco PIX 500 Series Security Appliances
5. Netscreen/Juniper NS series VPN Devices Screen OS 5.1.0 or above
6. Netscreen/Juniper SSG Series VPN Devices Screen OS 5.1.0 or above
7. Netscreen/Juniper ISG Series VPN Devices Screen OS 5.1.0 or above