I had a similar issue this is what i did to solve it
logged onto my online virgin account, went to plan and devices tab, scrolled down to the bottom and went to manage my services, scrolled down to voicemail, turned voicemail off and logged out. after 30 minutes took the same steps but turn the voicemail on and logged out. after 5 minutes from my mobile i dialled 222 and that setup my voicemail.
I am sorry to hear that the voicemail hasn’t activated properly on your iPhone 7. are you able to dial into the Voicemail? if you dial 07953 222 222 this should take you into the mailbox, if you get a message advising please contact the team it’s most likely that its locked.
@paperclip is spot on please give us a ring on 789 and we can have this reset or unlocked.