Ensure you are NOT in a Double NAT situation. ie. You do not have one router operating behind another router. For example, you want to put a Charter modem/router combo box into Bridge Mode, so that it is acting only as a modem, not a router, then, setup your dedicated router or firewall between the Charter modem/router combo box and the rest of the network.
Disable SIP ALG. Sometimes this is called SIP Transformations or otherwise.
Allow traffic to/from the following servers:
Name: IP Address:
Ringfree Server 1 22.214.171.124
Ringfree Server 2 126.96.36.199
Ringfree Server 3 188.8.131.52
Ringfree Server 4 184.108.40.206
On the following ports:
UDP, Ports 5060-5061 and 10000-20000
Where QOS (Quality of Service) options are available, prioritize traffic to/from those IPs, on those ports. Reserving 10% of the available bandwidth for phones is ideal, but not always necessary. At a minimum, reserve 120k of bandwidth for each expected simultaneous phone call.