Unlike Betamax/VHS/Video 2000 (Remember the Philips one that was best of the three and first to fall?), interoperability is possible for VoIP systems.
IMHO all the major telcos will soon be offering VoIP. Skype and Giz may grow big, may struggle and sell out, may buy up a big player in a reverse takeover, who cares. Crossover stuff, to call one VoIP system from another, will be taken for granted. Probably in a few years, the only landline handset you will be able to get will be an ADSL/VoIP thingy.
The eternal help vampire