Message on not enough credits

[massmux]

When creating a outbound redirection to an external number, for example an incoming call is redirected to a mobile external phone, all works fine but:

in case the customer has no credits left a message is sent back to the caller which sounds like "the call cannot be forwarded because no credit left" (or similar). Some persons are not comfortable with this and ask to remove it.

We cannot find a way to do so. Is there a way to disable that message?

thanks
Massimo