Watch out for Silent Call Guard, the alternative number listed for them goes through to someone who wont register you but just refers you to their 0870 number. 

Thanks for that.

I did a bit more research, and there was a report in an Oxfordshire local newsletter that suggested that they had not noticed any reduction in dropped calls when registered, and that it wasn't effective agains foreign companies.  Which is where, so i am told, most of these dropped calls originate from anyway.

Ah well...