19-01-2018 16:38 - edited 19-01-2018 16:41
@Anonymous 1750 to turn on and 1760 to turn off.
Also, if you want to increase the number of rings before your voicemail kicks in: https://community.o2.co.uk/t5/Pay-Monthly/How-to-Lengthen-Ringtime-on-any-O2-Mobile/m-p/362784#U362784
on 19-01-2018 16:39
Guide to VM here https://community.o2.co.uk/t5/Other-Products-Services/All-you-need-to-know-about-O2-Voicemail-901/m-...
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)