VoIP configuration via wireless modem/router

   A number of our customers reported problems of VoIP/SIP usage when they use VoIP over wireless mobile broadband (3G/4G) when an additional device like ATA (Analogue Telephone Adapter) or VoIP phone is connected to wireless modem by the Ethernet cable. Majority of these complains are coming from Optus mobile broadband subscribers (Optus E5186s-61a Router) and these problems such as: able to make outbound calls, but incoming calls are not going through or going straight to voice mail, or one way audio. In most of cases there were no problems at all with Telstra SIM in the same modem. Aggregating these problems we identified root cause of problem as a double NAT (Network Address Translation). VoIP SIP protocol has dependency on NAT and where it works fine at cable/ADSL/NBN - wireless may cause a problem because an additional layer of NAT is introduced. The fist NAT translation happens when wireless operator dedicates an internal address to modem/router and the second when external device is connected to wireless model.

   Unfortunately there is no single receipt how to fix this problem due to dependency on external network, outside of Australian Phone Company responsibility, however we found a few tricks that may help to resolve the issue. Unfortunately they are vendor dependent, may work for some people and may not help other.

   If your setup is wireless modem/router with an external VoIP device please try next steps to resolve problems.

optus wbb

dhcp modem

config phone2

sip alg

NAT Cone


If problem still exists you may try to replace your wireless modem by another model with built-in VoIP functionality. In this case you will remove a second NAT.