[Asterisk-Users] SIP Reorder tones

[Asterisk]

We have a strange issue here - we have the following setup:

Asterisk CVS-HEAD-12/15/04-07:42:16

40 SIP Cisco 7940 phones, linking to PSTN via EuroiSDN 30 channels.

Often, when someone tries to dial any internal extension or external 
number, they get the "Reorder" message. If they try again, they get 
another "Reorder" message. If they try a third time, the call gets through.

I've checked everything I can think of, but of course whenever I try, 
the calls make it through first time :)

Has anyone had anything similar before I spend hours trying to debug 
this one ?

Many thanks.

Julian