IAX2 call dropped when a second call comes in

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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