I notice this was discussed with regards to trunks, here:
and there is a solution dealing with the sip INVITE.
As a call to the voicemail system is, I believe, establishing a sip to sip connection from the extension to Asterisk, why not simply add an option in the Asterisk config file to have the voicemail system only offer a user selectable codec or set of codecs during negotiation instead of going off the list of allowed system codecs in asterisk config? The admin can then just specify voice codec for use with voicemail system. Then if the phone still sends video even after no video codec is available, you can mark this down as a braindead phone…
I was hesitant to mark this as a bug but it sounds like it is.
There does seem to be a hack in FreePBX:
Settings → Voicemail Admin → Settings → File Format and set the format to wav.