Panasonic KX-NCP500 Manual page 19

Is there only a single IP network between 2 ends of a call?
A huge degradation in speech quality will be produced when calls are made through multiple IP networks as
shown below; therefore, it is recommended that you avoid establishing a VoIP network in this fashion.
IP Network 1
IP Network 1
Document Version 2009-12
PSTN/
PRI QSIG, etc.
IP Network 2
2.1.2 Network Configuration
IP Network 2
IP Networking Guide 19