Perhaps you should rather ask this question the phone manufacturer. 

o2 just registrates the traffic.

Some phones are reported to have connected with the internet by themselves even if it was not activated in the menu.

Ypu could try and deactivate the wifi support in your phone’s menu. The wifi support might lead to a connection with the mobile internet if the wifi-signal is too weak.

If you wan’t to be sure to avoid such costs you can get the mobile internet deactivated completely in the o2-systems by contacting the o2-service.

Does “I did not connect to the Internet” mean that mobile data is deactivated in your phone? If so, contact o2 support. They’ll give you a refund if they find that little to no data was actually transferred. It’s a known technical issue related to the LTE network. Happened to me twice and I don’t even have an APN configured.

For reference (in German):

Unerwartete Datenverbindungen | heise online

@Atharva What smartphone you exactly use?

Its an iPhone 11 pro with iOS 14.2

Hi @Atharva ,

I can´t check the specific usage, I´m afraid.

However, I can see the SMS which notified you about the reached Limit of 30 MB.

Even though VoLTE can somethimes be mistaken for usage, as mentioned by @Maurice., 30 MB seem like a lot.

Too much, in my understanding, for just VoLTE.

Have you turned off mobile Data completely via the setting menue oder via the QuickSetting?

Viele Grüße,

Kurt