We’ll text you to tell you that you’ve used all of your SIM only allowance.
That means you won’t be able to go over your allowance without realising.
You can then buy an add-on to see you through til your monthly allowance renews.
Data add-ons
You can always buy more data to see you through to the end of the month. Just log in to My EE and go to Menu > Plan and add-ons (Pack and add-ons on pay as you go).