Hi, from the memory of the Planset that was installed where I once worked, I don't think the MAIN phone should ring when switched through to an external extension, the buzzer operates in sympathy with the ringing cadence instead.
I think that there is a modification to the strapping and ringing converter to allow the main to ring when switched through to an external extension however it may only be applicable to Plan 7, which only has one external extension and not plan 5 which has two extensions.
There's a load of information
here and the N diagrams can be found
here
Feel free to PM me if you require any additional help
Regards
Andrew