A PAN device in location A has an IPSEC tunnel terminating at a firewall in location B. There is a phone switch located behind the Juniper device. Phone users are unable to use standard 4-digit dialing to any extension within the system, though they are able to complete calls using 10-digit dialing.
If one side of the VPN tunnel is configured with a different MTU as the other firewall, this will cause traffic to and from the phone switch to be fragmented. This is usually interpreted by the software as a network issue and will reroute calls to the PRI instead of the proper route
Verify that both sides of the tunnel are configured for the same MTU to prevent fragmentation of packets. This will not only fix the issue in question, but will also improve general network performance across the tunnel. Fragmentation always causes latency so preventing it improves network speed.