Voicemail Always Asking for Extension Number?

I am using Linksys SPA942 phone with Sipx 4.0.4 installed. Voicemail works except that when I dial into from a registered phone it always asks for the extension number and password. It is my understanding that if it's a registered phone it should only ask for password. My setup is only being used on a LAN and phones otherwise registered ocrrectly and operate perfectly.

Is the provider register on the phone extension or registered on the spa942?
The problem may be something to do with this feature:
Cisco:
"Phone can be configured to carry a unique phone number or extension for each of its lines, or can be configured to use a shared number over multiple phones."

It's regsitered to a regular extension and not using any kind of shared line appearance or shared number

Perhaps the voicemail registration system is seeing the internal IP for the for the request and is registered (of course) to the external address or vice versa.