Putty has an option to log all information displayed on the putty screen to a log file:
Right-click on the Window and select 'Change Settings'.
Top left under 'Sessions' click on 'Logging'.
Then under 'Logging Session' tick the 'All Session Output' option and APPLY.
You can configure the output file name and location. This file will show us if any 'funny' characters is send off by the PABX unit, etc.
NOTE: to have access to the attachment, you need to be registered and logged in.