Looking harder voip actually seems more expensive when you go above 5 users. As there is a £10 license perhandset for a hosted solution per month
If you buy kit you pay a licence fee per port even adding an extension costs money. After 3 years the extra cost of Pbx and isdn line install is offset by the cheaper upfront but higher ongoing charges for voip. Currently I can't see the point, unless you save a lot of money on site to voip, in which case why wouldn't you just do it internally anyway