My old phone doesn't work with ntl

Got a phone/broadband package put in with NTL that came with a free phone. As I wanted to keep my old number from eircom, NTL gave me a new temporary number while the old number was 'ported' and this 'porting' was completed a couple of days ago so I decided to unplug their phone and put in my old phone as this has an answering machine but I can't get a dial tone or it doesn't ring when the number has been dialled (the NTL phone works perfectly). NTL assured me the phone would work and the phone is less than a year old.
P.S. I have to now plug the phone into the back of the modem/router

Any suggestion on how to fix this or why it isn't working?
Cheers.

that phone from ntl was a voip/digital phone. i assume your original phone was analogue and thus to use you'd need a ata converter (eg [broken link removed])

S.