3 channels over VPN


I have a problem with the quality over dsl

I have two offices.
In one office is asterisk with two phones (lets say office1)
In the other office are 4 phones (office2)
The two offices are connected with vpn over dsl with
zyxel 660 routers

Zyxel has a generic VOIP-SIP “QoS” which seems to work somehow and i have installed an experimental g729 codec.

when i have 2 phone calls between office1 and office2 it works fine
when i try for a third call between two phones both of them in office2 the voice has lot of interrupts and not good quality while the other 2 phone calls still work fine (no quality problems)

  1. On the third phone call the rtp packets suppose to be peer2peer between two phones (LAN), so why the interrupts and bad quality between 2 phones on the same LAN?

  2. in case “nat=yes” for the client do the rtp packets go through the asterisk server?

  3. the dsl lines are not used for other porposes. Are zyxel router strong enough to handle RTP packets? (cpu is not over 15% as it shows). Is it possible that the switches cannot handle the rtp packets on the lan?(!)

  4. any hints what causes the problem or how i fix it?

Thank you