Primus GRE Setup when using a Cisco 1841 Router (T1,Fiber, FTTN)

The GRE is a configuration in your HPBX phone system's router that will automatically re-route your voice traffic through your data connection, or dedicated backup connection, should the primary Internet connection for the HPBX phone system ever go down. This is so that you can continue to place calls while we repair the Internet connection for the HPBX phone system. 

To start the process, an Ethernet cable is to be connected on Primus POE switch from G2 or G3 (if G2 is being use to connect another POE switch) toward your DATA network modem/router or Firewall.

For us to configure the GRE from here, we required the following IP address info (with shared Public IP), which is 90% of cases:

1- Public IP of your Internet Connection (must be static)
2- IP to assign to the HPBX 1841 Router (must be static)
3- IP of internal Gateway
5- Subnet Mask of Internal Network

These settings need to be configured in your firewall: 

1- Enable GRE (Protocol 47 - GRE is protocol 47 not port 47. On most routers you are looking for "enable PPPTP pass-through"; some it is simply VPN pass-through, others it is not supported)
2- Open port 500 UDP (IKE)
3- Open port 4500 UDP (NAT-T)
4- IP address to be white-listed: (East) & (West)