[asterisk-users] SIP to IAX2 with delayed echo

[Dave Platt]

> Coming from outside the network, setting up for a couple rounds of
> NATting isn't going to work well.  They are not seeing it between
> phones.  Others, using the polycom phones have reported echo between two
> SIP on a 4ms ping trip.

Could this be due to a purely acoustic echo within the Polycom handsets?

I encountered a nasty echo / hollow sound when using a cheap USB
"telephone" to connect to my Asterisk system (via KPhoneSI).  The
echoing was due to acoustic feedback - the handset body acted as a
very nice channel for sound waves from the back side of the
speaker down to the microphone cartridge.

I opened up the handset, added some damping materials (panel-
vibration-damping and soft-foam sheeting, left over from a
car stereo speaker installation I did), closed it back up,
and the echoing was gone.

You might not notice in some calls, if the Polycom phones have
silence-detection turned on for those calls and if the amount
of feedback falls below the phones' silence threshold.  If
the phone silence-detection algorithm were turned off on
other calls, the echo would then be audible.