Provides an optional regex expression that will be applied against the agent callback number. If the callback number does not pass the regex filter then it will be treated as invalid. This can be used to restrict agents to extensions within a range, not allow callbacks to include keys like *, or any other use that may be appropriate. An example input might be:<br />^([2-4][0-9]{3})$<br />This would restrict agents to extensions 2000-4999. Or <br />^([0-9]+)$ would allow any number of any length, but restrict the * key.<br />WARNING: make sure you understand what you are doing or otherwise leave this blank!