Hi @Dewdrop1
Is this Voicemail ?
Voicemail is driven by your network with the messages held on their server until they are deleted usually by the account holder.
Your phone wouldn't in my understanding delete Voicemail messages.
I'd suggest to speak with your network support department.
To add unfortunately if they have been deleted off the Network's Server then they'll be no longer available to pull back.
I wish you all the best. 
