Treat voicemail number like an implicit contact

This makes it so the voicemail number is treated kinda like an implicit contact. "Voicemail" is set as the call contact alias when in a voicemail call. As a result, the voicemail number is checked more often and is memoized after the first time it is checked.

I haven't updated the call history page to display voicemail calls as such due to some issues with the voicemail info not being available right away.