VoIP implementation guidelines
Almost all issues occur due to incorrect configuration of infrastructure and/ or hardware. The following items apply as a generic guide and should be observed in all situations, regardless of the hardware used.
General
Avoid NAT behind NAT at all times.
SIP functions (e.g. SIP ALG) in the router must always be switched off.
Do not use STUN, TURN, ICE, or UPnP.
Switch on Keep-A-Live (10 seconds is a good value).
Port Forwarding is not necessary and is not recommended.
Preferably use a dedicated internet connection for VoIP to rule out interference from other data traffic. Contention ratio (1:1).
Dimension the data line on the basis of 0.1 Mbps per call (up and down) when using G.711 A-law (PCMA), or 0.033 Mbps per call when using G.729.
Make sure the MTU in the modem/ router is correctly set for the data line used.
Use a separate network for voice and data (VLAN).
Configure QOS in switches and routers to guarantee bandwidth and prioritise voice traffic.
Always use hostnames for registration. IP addresses may change.
The proxy domain supports SRV records.
SIP/ UDP is preferred over SIP/ TCP.
Preferably use G.711 A-law (PCMA). This gives the best sound quality. Where bandwidth is limited G.729a may be considered.
DTMF support is based on RFC2833
Hosted PBX
Preferably use the G.711a (PCMA) codec with a packet size of 20 milliseconds
Take advantage of unique local listen port for each SIP device (5061, 5062 etc.)
A good NAT router is important for hosted VoIP telephony. Good examples: Cisco 800/1800, Mikrotik (Router Board)
Your own IP PBX
Preferably do not install the IP PBX behind NAT. If it is desired to do so, place the PBX behind NAT 1:1 with an unused public IP address.
Avoid dynamic NAT. This often causes problems.
Provide the IP PBX with a firewall that only allows traffic to and from a restricted IP range. More info…
Let the IP PBX authenticate/ match on account_id and not on hostname. Incoming traffic can come from multiple hosts and therefore can be denied.
A static trunk is more reliable than a dynamic trunk with registration.
By default DID/ DDI is set to the full number (including country code with a ‘+’ prefix). This can be changed in Freedom if desired.
Outgoing CallerID has the same format. (+27210123456)
Fax
Recommended packet size for G.711 A-law faxes is 10 milliseconds.
For fax always use T.38.
Echo cancellation must be switched off.