Hi JamieF22,
Thanks for posting and welcome to our community 🙂
Sorry to hear you've been having some issues with the voicemail, there are a couple of things you could try-
Can you try leaving a new voicemail on the phone. Once you've done this, you must listen to the new voicemail in full until you're prompted to delete it, at which point you can go ahead and do this.
If that doesn't work, can you check your voicemail inbox is definitely empty by deleting all messages, including any saved ones, and then perform a reboot (not a factory reset) of the handset.
Alex_Rm