What is this code for? *100#

[Xefo]

Someone dialed *100# from my mobile and got (Done) message. I really worried. What did he do?! What is this code for?

[Bambino]

@Xefo Doesn't produce anything from a Google search. Call customer service if you're concerned.
Call 202 from your mobile or 0344 809 0202, 0800 902 0217 (Payment Management) or 0800 032 1402
When it comes to the reason you are calling, stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say 'Upgrade', 'Lost/Stolen', or 'Fraud' if the suggestion above doesn't get you through.
Best time to call is 8 am.
You can also reach O2 via social media:
Facebook (https://o2uk.co/O2CFB), Twitter (now X) (https://o2uk.co/O2CTW), or Instagram (https://o2uk.co/O2CIG)

[MI5]

@Xefo 

It's the code to get your PAYG balance on the Vodafone network so won't do anything on O2.