asterisk users - Jan 2008 - remote Snom 360: no voice passing thru

Hi,
I have an Asterisk PBX 1.2.18 connected to a remote Snom 360 (firmware 
ver: 6.5.10), outside my LAN. If I call the Snom, I sometimes cannot 
hear the called party and the called party cannot hear me. When this 
happens, I get the following message from Asterisk CLI:

NOTICE[19164]: chan_sip.c:2012 auto_congest: Auto-congesting 
SIP/3-081da588  -- SIP/3-081da588 is circuit-busy

I searched on internet but found nothing (something similar was an old 
bug but now fixed).

Is there anybody who knows what is causing this strange (random) behaviour?

Consider that:
- there is no firewall between Asterisk and the Snom (Asterisk <-> 
router A <-> internet <-> router B <-> Snom 360)
- I have a Grandstream ATA connected to the same router where the Snom 
is connected but the ATA works good.
- I tried to change cables without success
- rtp debug shows no voice packet is sent between Asterisk and Snom

Any help is appreciated.

Thank you.