I notice that most providers use SIP now, with the exception of skype
propietary system.

Do any providers provide support for a H323 phone ?
or do I need to provide a gatekeeper or PABX on my network to route out from
a H323 phone ?

I've acquired a H323 phone and wondered what I could do with it - I was
going to try running a PABX on my PC anyway but it'd be nice to be able to
use it with the PC switched off.