Can't remove phone number in O365 (local sync)

Hi all,

I'm having a fun little problem where for some users a phantom phone number is appearing in our GAL and their O365 profiles, this number can't be removed on the O365 side as it's giving the usual error that it's being synced from the local AD but trawling through ASDI there's no reference to it at all. Honestly i've been banging my head agains this and can't find where this dammed attribute could be living. This is what it looks like on the O365 profile.

Any help would be very much appreciated.