asterisk users - Oct 2010 - IAX2 call dropped when a second call comes in

Hello list,

I have this problem with dropped calls on Asterisk.

The setup is SIP internal extensions (Grandstream GXP-2000), two 
internal analogue DAHDI extensions and IAX2 trunk lines. IAX2 trunks use 
ulaw/alaw. The Internet connection is ADSL. Asterisk is 1.6.1.6

Everything worked fine until about 1.5 months ago (for 1 year) until the 
client started to report dropped call. The scenario tends to be:

1. Client is on an external call (through trunk).
2. Another call comes in.
3. As soon as the second call is picked up, first call drops.

I thought it might be a bandwidth problem - so I checked upstream and 
downstream bandwidth. The smallest one is downstream - at about 300kbs. 
I'm not sure they ever have more then 2 trunk calls at the same time.

I have turned logging to verbose in logger.conf, but I just can't see 
anything that seems relevant in the logs. I can attach the parts of the 
logs during which I've been told by the client the calls dropped if 
anyone would like to have a look at them. I can also attach iax.conf or 
any other config file if you would like to see it.

The provider is adamant that there is no problem at their end.

Any ideas on this one would be much appreciated.

Sebastian