Strange call behaviour


I have users complaining about calls that get crossed with other channels and/or that get dropped or switch to hold music for the both callers. I have asked them for more information, here is the call log for one of the calls they gave me.

This is what happened in the call:
outgoing call from extension 10 using PSTN (over linksys voice adapter)
call forwarded to extension 11
call forwarded to extension 13
they talk for about 5 minutes, then exension hears another call join the conversation, extension and the PSTN call drops.

I have the log, but it is over 1000 lines, so i am not sure if i should post it

I am new to this forum, if this is not the correct section (or forum) please let me know

thanks for your help