Call 333 from your three mobile or  0333 338 1001 from any other phone

select 4 in first voice menu

select 5 in second voice menu

wait for operator and ask him to switch it of or on

have last 6 digits from your SIM ready as he will need this to verify you

From Three Mobile Wewbsite

http://support.three.co.uk/SRVS/CGI-BIN/WEBISAPI.DLL?Command=New,Kb=Mobile,Ts=Mobile,T=Article,varset_cat=callsemails,varset_subcat=3794,Case=obj(3354)

Turning off voicemail. If you want to turn off yourvoicemail, you’ll need to call us on on 333 from aThree phone or 0333 338 1001 (standard rates apply) from any other phone. Once you’ve turned off voicemail, you’ll have to call us again on the same number if you want to set it back up again.