I can't imagine that VOIP (Voice Over Internet Protocol) ever worked worth a hoot on a dialup connection. Anyway, this is something that requires software designed for that purpose. So, again, what are you using for this?
I've used Yahoo Messenger for phone calls via PC and it, i.e. the software, has the number pad feature. So, that's what I mean, it would be part of the software you use for VOIP.