Hi Gerard,
You will probably have success with using TryParse rather than Convert:
Dim x As Integer
Integer.TryParse(rdrCount("CALLS").ToString, x)
The TryParse() method returns a boolean value, which you could also test. What I showed above will work just fine also. The integer x will be zero if it could not parse the string (so, if the string was null or empty).
I hope the online converter that I used worked correctly though. The integer parameter in the TryParse() method is an "out" parameter in C# ... the converters didn't translate that at all to VB code. But, anyway, see if that helps.
~~Bonnie