yeah, it doesnt make much sense from US mobiles in general because you pay inbound charges. However if you have a lot of minutes to burn anway it might be worth it :)

I'm using the Android version, and not seeing the option for multiple numbers.

It seems that you can log out and create a different account with a different number.

The catch would seem to be that if I create an account to call back and put money into that account, if I then get a new SIM the money is lost.

Am I missing something here?

There's a free Android app call DialMax through which you can trigger "phone-to-phone" callbacks for all Dellmont VoIP brands.

I just downloaded and installed based upon your recommendation in another thread. Seems to work well.

Easycallback has very attractive rates-but, if there is no portability from SIM to SIM, it is not for everyone.

MobileVoip Now Works With Callback
 

I just downloaded a new version of Mobilevoip from the Android Market (ver. 3.11.) The callback is now clearly implemented, and there is a whole new (better) interface. I believe that this product works with all of the Betamax/Dellmont/Finarea services. It does not ask you which service you are using, it just knows based on user and password.

It also shows you the billing for the call before it puts the call through - i.e., the cost for each leg, including any connection fee. I found it to be very reliable in my testing.