There are some circumstances that may warrant the use of randomized SIP Listening ports. Such as to work around Firewalls with poor NAT Translation. This effectively changes the port that phones receive SIP traffic on, they will still send to the PBX at port 5060.

In a normal scenario (restricted cone NAT), all of your phones will listen for SIP traffic on the default SIP Port of 5060; when you have more than 2 phones on a network the first will get a public port of 5060, the second phone will get a randomized public port. Both phones cannot share the same public IP and Port unless there is some kind of ALG. Dynamically changing the SIP listening port of all of your devices will prevent your firewall from having to do the port translation. 

Example without Randomized SIP Ports: Phone 1 and Phone 2 use the standard SIP Listening Port of 5060. Phone 1 uses public IP and Port 12.34.56.78:5060, Phone 2 uses public IP and Port 12.34.56.78.1031. The firewall knows that anything received on the WAN IP and port 5060 goes to Phone 1, and anything received at port 1031 goes to Phone 2.

Example with randomized SIP Ports: Phone 1 gets a random SIP Listening port of 2395, Phone 2 gets a random SIP Listening port of 1032. Each of these phones will use these ports on the Public (WAN) side of the firewall if there are no port conflicts. Phone 1 uses public IP and Port 12.34.56.78:2395, Phone 2 uses public IP and Port 12.34.56.78:1032. The firewall knows that anything received on the WAN IP and port of 2395 goes to Phone 1, and anything received at port 1032 goes to Phone 2.

Overrides

These overrides can be used as Domain or Brand Specific overrides, the will NOT work as Device level overrides. There is a variable [[dynamicPort]] that will generate  a random port for each phone.

Yealink

sip.listen_port="[[dynamicPort]]"

Polycom

voIpProt.SIP.local.port="[[dynamicPort]]"

Vtech ET/Snom D series

Vtech ET Series and Snom D Series phones utilize random SIP listening ports by default.

Grandstream GXP Series

P40="[[dynamicPort]]"


What situations will random SIP Listening ports help with?

Confirming there is a NAT issue at the firewall requires looking at LAN and WAN packet captures and seeing how the packets are being delivered. In general, randomizing SIP Listening ports will help in scenarios where the customer's local Firewall is preventing normal functioning BLF or MWI.