If there is excessive amounts of network traffic then Axon May think Express Talk is busy and therefore send the call straight to VoiceMail.
The computer with express talk may be really busy at the time and then have the same effect as the first scenario with Axon not getting a reply fast enough.
I gather your using the setting 'transfer to voicemail if unanswered or busy'
What you could do to possibly get around this is put the extension and voicemail into a group and set the voicemail to start ringing after a set number of rings. then if the computer with express talk isn't found straight away, it will still have a chance to start ringing if axon finds it before the voicemail starts ringing.
Try Updating to Axon 1.07 if you haven't already done so.