manula,
Dial Plans don't come into play using Web Callback. It would appear you are doing everything correctly, especially since there is a common rash of identical problems with this family of providers.
If you can in fact make simultaneous outgoing calls on the same VoIPDiscount, VoIPStunt, and SipDiscount providers via Asterisk, it would appear there is some technical issue between Web Callback and these providers.
Martin, can you take a closer look at this particular scenario and see if you can determine what's breaking down?
Ron
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