Hi @Mooky,
Welcome to the community,
All voicemail numbers should be attached to the SIM card, you may need to import the contact from the SIM within the Contacts section.
Contacts > Menu Button > Import/Export > Choose SIM > Choose Phone Contact > Check to see if voicemail is the list.
If you cannot find it, you may need to contact your network carrier and find out what the number is.
I hope this is the information you were looking for.
