The voicemail number can be changed in FreePBX using "My Voicemail" in the feature code page. Many phones have a dedicated voicemail button. A significant proportion of these determine the number to dial when the button is pressed by looking at the message-account information in the sip voicemail notify message. This is configured in asterisk using the vmexten parameter in sip.conf. There is currently no linkage between the vmexten settings and the actual voicemail number set in the feature code page.
I propose that featurecodeadmin/functions.inc.php look for the special case of feature name "myvoicemail" and add it to the sip table of the sql database using the ugly 9999999999999 technique. retrieve_sip_conf_from_mysql.pl will then implement it in the right part of sip_additional.conf