Recovering voicemails after a carrier change?

I’m not hopeful, but is there any way of recovering voicemails if you change carriers/providers?

I’m in the UK, and swapped from O2 to iD mobile on my iPhone 7 just a few days ago. I didn’t even think, and now I’ve lost all the voicemails I had from my mum, who passed away in July. I stupidly didn’t have back ups, just the visual voicemail on my phone.

Everything I’ve read says that they will have been automatically erased from the O2 servers, but one page mentioned the use of either PhoneView or iExplorer to potentially recover them? Any help at all would be greatly appreciated, the thought of never hearing her voice again is breaking my heart.