Parked calls joining with external call

I’ve tried desperately to solve this alone, but have failed miserably and cannot find any forum posts with similar problems.
We have a client with the following issue:

User A is on an external call (Caller 1) and another call comes in (Caller 2)to User A’s ring group.
User A uses call parking to park Caller 1
User A replaces his receiver so the ring group sees him as available and starts ringing his extension
User A picks up his receiver and rather than getting Caller 2, Caller 1 is joined with Caller 2 and User A has nobody.
The two external callers can then talk to each other (not ideal!) until they hang up, and neither call can be retrieved from any parking lots.

The system is running:
Asterisk 1.4.39
Dahdi 2.4.0
Sangoma PRI card using UK ISDN30
Built originally using PBXIAF distro

I’m happy to provide chunks of log file, just let me know which ones might help.
Many thanks in advance.


Any comments or suggestions would be greatly appreciated - the client is talking about ditching the phone system because of this :frowning: