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Bill My Calls

The VoIP problems associated with NAT and solutions to those problems.

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NAT - one way audio

NAT (Network Address Translation) is a mechanism used by routers to share a single public IP address between devices on a private network by allocating each device with a private IP address.  If the computers on your network have IP addresses in the following ranges, then your router uses NAT:
  • 10.0.0.0  - 10.255.255.255
  • 172.16.0.0  -  172.31.255.255
  • 192.168.0.0  -  192.168.255.255

NAT is a valuable tool that allows only one public IP address to be allocated to an internet connection, rather than one address for each computer.  It works flawlessly for most internet protocols, but often causes problems with Voice over IP.  If you are affected by this problem, then you will experience one-way audio.  Your callers will be able to hear you, but you will not be able to hear them, because your router is blocking incoming audio packets from reaching your VoIP device.

Testing for NAT problem

The simplest way to check whether NAT will cause you problems is to make a call using your chosen VoIP carrier.  If you have two way audio, then there is no problem.  If there is no NAT problem when your VoIP gateway is registered directly to your VoIP carrier, then there should be no NAT problem when your calls are routed through the call shop billing service offered by Bill My Calls.

We offer two test announcements on our network.  If you configure your VoIP device to register itself on the Bill My Calls system, you can call these test numbers:

  • 123 is a test announcement that includes a NAT penetration service.  We send the audio back to you in a way that should bypass the NAT problems introduced by your router.   You should always be able to hear this announcement.
  • 124 plays the same announcement, but does not use NAT penetration.  If your device is behind a NAT router, you may not be able to hear this message, unless your router makes special provisions for voice over IP.

If calls work correctly with two way audio then you do not need to make any changes.   But, if you have one way audio from your ITSP (VoIP carrier), then there are three solutions available when you use Bill My Calls: carrier NAT penetration; router configuration and Bill My Calls NAT Penetration.

Carrier NAT penetration

Many carriers offer a NAT penetration service identical to the system we use on our 123 test announcement.  This is particularly true of carriers that sell into the retail market.  Wholesale carriers may not offer this service on their standard accounts, but may be able to enable it on request.  Find out if your ITSP supports "NAT penetration" or "Symmetrical RTP" and ask them to enable this feature on your account.

Router configuration

You can often fix this one way audio problem by reconfiguring your router.  If you only have one VoIP gateway, then the easiest solution is to look for a feature called DMZ on your router.  Enable this feature and set it to the private IP address of your SIP or H323 gateway.  This will cause all unexpected packets to be be forwarded by your router to your VoIP gateway and should result in two way audio.

If you have more than one VoIP device, then the situation is more complicated.  You will need to set up individual port forwarding for each device.  You should make sure that each VoIP gateway uses a different range of RTP ports.  You should then look for a feature called port forwarding on your router and set this up so that each device's range of ports are forwarded to that device's private IP address.

You can check if you have fixed the problem by calling our 124 test announcement.   If you can hear the announcement, then you have fixed the problem.  It is a good idea to set private fixed IP addresses for your VoIP gateways rather than letting them use DHCP.  If, through DCHP, the private IP addresses of the VoIP devices change, then your DMZ or port forwarding settings will become invalid.

Bill My Calls NAT penetration

If these two methods fail, then we can provide a NAT Penetration service through Bill My Calls.  The service can be requested by sending an email to our support team.  But, we charge USD 0.0040 per minute for these calls, so it is best to explore one of the other solutions first.
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Last modified: 21 November 2008

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