I have an old 300 series rotary (pulse) dial phone which has been fully restored and which I had hoped would work on my VM/NTL line - but it doesn't

I get a dialling tone OK, but this remains there as I dial a number. Others on this thread are quite definite that it is possible to use a pulse dial phone on a modern line - so where am I missing a trick?
The old phone is very nice as an ornament/antique - but it would be even better if I could use it.