I think that the most important things when choosing such product is reliability, scalability and inter-operability. (the 3 Xabilities...)
If you have good experience with VocalTec I would choose them rather then trying the cheap option.
VocalTec has been in the VoIP industry forever, in fact if I'm not mistaken, I think they have even invented it.
But that depends on your budget and decisions.
Is this the product they offered you?
SS7 SIP voip gateway