Jajah bridges two regular phones and use VoIP for low cost transport.

To use Skype, you need a microphone but the rates are very cheap so many people have invested in one.

With JaJah you don’t need a microphone but you probably pay a little more because they have to go through the PSTN on both ends.

When JaJah says they have 10M users, I doubt that they mean 10M users who use their service as actively as Skype users use Skype. Else, they’d publish the volume of traffic that passes through them, not the number of users.

95% of all VoIP traffic belongs to Skype.

So you’re left with 5% room for Jajah, all the J-something services, and the large number of VoIP transport providers out there who often carry traffic for wireless providers.