The number you are calling is it a landline or mobile, as from what you describe I can see that this is related to the mobile number being called has no credit, if the line rings then the call is going through but isnt being connected to the number.
The issue would be down to the receiving end and there is nothing that VM can do about that.
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We think we know the cause in our case. The person that we were trying to call ( a landline) had their phone turned off. If you've come across this message in the circumstances you describe, then this message sounds like a generic " can't connect you for some unknown reason" type message. Not very helpful.