Outgoing calls not going to voicemail

I am using callme.php to instigate an outgoing call to a phone number and to play back a message. If the phone is not answered asterisk logs the call as ANSWERED in the cdr file, but does not leave a message on the phone’s voicemail.

I notice in callme.php there is a WaitTime - does anyone know if this determines how long to wait before hanging up? If not, does anyone know how to get around this problem?