Hi Riccardo.
There is another method you could try which won't involve changing any of your internal wiring.
I have a Planset N625 wired with 2 "external" extensions. These extensions are 2-wire 706 phones effectively wired in parallel.
What the GPO did to eliminate bell tinkle on those was fit a thermistor in series with each bell coil. This causes a slight delay with the phones start to ring (about 1 second) but does stop bell tinkle.
The device you are looking for is a
Thermistor 1A-1. These are available from eBay and other GPO spares suppliers.
You would wire it in place of the strap across T4 & T5 is each 706 telephone. You may already have a resistor in place of this strap. If so then post a picture of the wiring of your phones and I can advise how it can be done.
Hope this helps.
Regards,
Wayne