Unfortunately when you change contracts your old voicemails are deleted off the O2 server.
There is no way to recover them.
They should have been backed up first by yourself I'm afraid.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)