[netghoster]

Haven't heard too much about it? Nobody want it?

[Matrix Leader]

I use VYKE MOBILE! Awesome!

You can even set it to pop in automatically when you make a call or only activate when you make an international call.

Google VYKE

[Dubdub]

I do not know of a VoIP client for BB. If VYKE Mobile works as mentioned above, it would be the first.

Plus it doesn't appear to be free either.

[lmcjipo]

There are 2 VoIP clients posted by people working at the 2 companies in the "Aftermarket Software" forum/thread. One of them is called BlackVoib (currently in open beta) and the other is called VoIPSwitch ($30 USD which doesn't seem to have a trial/demo).

[14pups]

Here's another: TringMe

Mobile VoIP Comes to BlackBerry Devices

[Matrix Leader]

Vyke isn't free but it integrates smoothly with the phone. U can even set it to launch automatically when placing an international call

[robmi]

and another one: fgVoIP from fgmicrotec. Generic SIP VoIP client. Works with any SIP PBX and VoIP provider
shop.fgmicrotec.com