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 a wireless modem by the Ethernet cable. The majority of these complaints 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 cases, there were no problems at all with Telstra SIM in the same modem. Aggregating these problems we identified the root cause of the problem as a double NAT (Network Address Translation). VoIP SIP protocol has a 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 first NAT translation happens when the wireless operator dedicates an internal address to the modem/router and the second when the external device is connected to the wireless model.

   Unfortunately, there is no single-receipt how to fix this problem due to dependency on the 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 others.

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

• Connect your VoIP phone/ATA directly to the wireless modem (Yellow Ethernet port usially), please make sure that there are no intermediate routers/switches.

• Ensure that DHCP server is enabled in wireless modem and your VoIP device expects DHCP

• Register your account with standard username/password settings, but enable TCP transport network protocol on your VoIP phone or ATA (Not all devices support it) instead of default UDP (Example on Yealink SIP Phone)

• Register your account with standard username/password settings, but enable TCP transport network protocol on your VoIP phone or ATA (Not all devices support it) instead of default UDP (Example on Cisco SPA-112 Phone adapter)

  

• DISABLE SIP ALG at your wireless modem (Remove tick on "Enable SIP ALG")

• Define and enable STUN server as stun.australianphone.com.au in your VoIP device (not available in all models)

If the problem still exists you may try to replace your wireless modem with another model with built-in VoIP functionality. In this case, you will remove a second NAT. Also, please make sure that your phone/ATA box is connected directly to the wireless modem/router by the cable, without any intermediate devices (USB Dongle to the computer and phone to computers' Ethernet cable is not working solution as well, unfortunately).