dustXman, I set up Gizmo to work with my existing SIP-accounts (sipcall.at and sipgate.at being the providers) without any problems.

The only thing you need to know is that you need to set up your existing account through the "secondary account"-option on the login screen. I never found any other method for this from within Gizmo. Also, Gizmo never uses the axpression "SIP-account", they always refer to it as "secondary account". (This is probably because Gizmo itself uses SIP itself, so technically "SIP" doesn't make a difference )