Well after a lot of searching it seems that one company in the US does indeed make such a unit, they call it the Pulse to Tone Converter, it can be seen here about 3/4 of the way down the page:
http://www.sandman.com/digit.html
Oddly enough the reason that I want this is mobile phone related, I found a unit for sale on eBay which allows you to connect a regular telephone to the mobile network, but most of these units seem to only accept DTMF, I want to be able to use a 200 or 300 series telephone as a car phone, and so using the 12 volt supply from the car this is easily possible, all that is needed is the pulse to tone converter.
I have ordered one of these American units, so it will be interesting to see how well it works.
This is the type of unit I purchased from eBay (this listing is in no way related to me, I do not know the seller):
http://cgi.ebay.co.uk/ws/eBayISAPI.d..._promot_widget
It seems that the use of vintage phones on the mobile network is possible, which is great news. Just imagine a candlestick phone as your car phone!