Starting another application from a "callto" link

Dear all, Does anyone know how to get any other application other than Skype (SJPhone, for example) to start and call a number once a user clicked on a "callto" link? (Win XP, IE and Firefox) How should a callto link look like?

Now any time I click on a callto link, Skype starts and reports that the username doesn't exist.

Any ideas?

Thanks a lot, Roman Zhovtulya