If you have any old British telephone (pre 1989) and convert it to the new UK phone system it will NOT be officially compatible (as a modern phone would be).
However some old antique telephones can be sympathetically converted so that they will still function properly when connected to a modern telephone exchange should you wish to use them (unofficially). This does not however mean that converted phones are approved by BT.
Converted GPO dial phones currently still work on outgoing calls on BT lines (including BT broadband). However pulse dialling is slow when you are used to tone dialling. If you dont use BT please check with other telecom providers and ask them if they support pulse dial telephones.
It is also very likely that all phone providers will stop supporting pulse dialling at some point in the future (no dates yet). However when this happens we believe the bells will still ring and the phone should work OK for incoming calls, this should not change in the foreseeable future.