This will fit your any genuine GPO model 712, 722 Series Trimphone including all Snowdon models either push button or dial.
Fitting instruction will be supplied and this required no soldering or modifications. Simply remove some parts in the handset and use existing handset cord terminals. Keep the parts removed in a safe place if you ever want to reverse the operation without any damage having being made at all
No special skills required to fit and will take around 10 minutes.
In my opinion a Trimphone is not usable for everyday use without one of these as they crackle and hiss and fade in and out with the carbon one fitted. The sound quality should be brought up to modern standards.