Voicemail divert number not working

[ErricoM]

I am on a sim-only monthly contract. I don't use a smart phone. I use a Nokia burner phone. I am trying to divert all calls to my voice mail. It asked me for a number and I entered 901, but this was not recognised. I have called 1750 to make sure that my voicemail is active. It still doesn't work.

I suspect I need a full voicemail number. Does this exist? If so, what is it?

[Enlli]

If it requires the long number try dialing 1780. That gives the number for use abroad. But read here first

https://community.o2.co.uk/t5/How-to-Guides/All-you-need-to-know-about-O2-Voicemail-901/ba-p/547712

[ErricoM]

Thank you for the suggestion. It doesn't seem to work. I've also tried entering the voicemail abroad number and my own number in various formats. This also doesn't work. There must be a dedicated long-form mail box number.

[Enlli]

Have you tried  Divert all calls with the code
 ** 21 * 901 # SEND 

Best done when not on 4G

[ErricoM]

That doesn't work either. It gives the same "request not completed" message as when trying to input 901 etc into the call divert number. 

I had this problem with Virgin but their long-form mailbox number is easily found. I'm certain that I need one for the o2 mailbox, I just don't know what it is.

[Enlli]

Try 

07802091901