[Freeswitch-users] softphones on Amazon

Steven Ayre steveayre at gmail.com
Mon Jan 28 13:12:34 MSK 2013

One way audio usually is a result of NAT.

Not all NAT routers operate in the same way, and not all softphones handle
it in the same way, so that may explain why it works in one location but
not the other. I assume the softphones were behind NAT?

Set the FS log level to debug, and check the IP in the remote SDP. You need
the phone's external IP in there. Phones usually determine that using STUN.
Some NAT routers have a feature called SIP ALG to rewrite it for phones not
doing it themselves, but that often will conflict with phones that have
already put the correct IP (and also cannot work with TLS).

In short check where RTP is being sent to is correct...

Without the correct IP FS also has a last-resort feature where it detects
the IP the caller sends audio from and replies to there (RTP auto-adjust).
A log entry indicates when that happens. But that requires FS to receive
audio, which means it can't work until the caller starts speaking after the
call is answered (so might miss hearing the callee say 'hello'), and the
caller will never hear ringing (unless generated locally by the phone when
it gets the SIP 183/180 response).

The NAT page on the Wiki may be informative:

Just to complicate things Amazon EC2 runs behind a NAT layer too AFAIK. I
don't have any personal experience with EC2.


On 28 January 2013 09:26, Stuart Mills <stuart.mills3 at btopenworld.com>wrote:

>   Hi,
> I have a strange problem using FreeSwitch on an Amazon EC2 instance and
> was wondering if anyone out there could help me.
> The problem seems to be that I can register an extension from a remote
> network in say France, phone works ok but when passing calls to it (from
> sip peer through JavaScript to user extension) I get one way audio, the
> caller can hear me on the softphone but I can’t hear them – like the
> reverse audio path can’t be established.
> That would be ok and fixable if the issue was consistent but on another
> phone registered from within the UK, it all works fine and I get a good 2
> way conversation.
> If I dial from one softphone in France directly to the other in the UK,
> that works too.
> I followed the wiki’s guide to installing on Amazon and have everything
> right, otherwise I doubt it would work at all.
> Is anyone able to help me or have experience of this type of issue? Maybe
> it’s a local router issue to the phone in France but I didn’t need to do
> anything special to the one in the UK to work, so not sure it could be that
> either.
> Kind Regards,
> Stuart
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