We would hate to see you leave so please get in touch if there is anything we can do to sway you, but if you really have to...
Open the Side menu and scroll down to Settings, then scroll down until you see “Delete Account”. Once you click, you’ll be asked to confirm your account deletion.
N.B. This cannot be reversed and you’ll lose all numbers and associated data.
Once you confirm, your onoff App account will be deleted.