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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. |
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| 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. |
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| 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. |
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| 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. |
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Bill My Calls
NAT penetration
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| 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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