Nothing fancy, and I sent it to the contact point they have in their privacy policy. You can find it on their websites (or at least I found it; in my case it was just an email but it will possibly escalate to snail mail). As far as contents goes - here it is, translated into English:
I am a/an [OPERATOR] subscriber, identification data: (...)
I would like to get a complete list of personal data that [OPERATOR] stores about me:
- The history of telephone calls, metadata (when and where [to which number] I called or when and who [what number] called me) as well as the data itself (the content of the calls themselves);
- The history of text messages, as in the case of calls - metadata and the data itself;
- History of data transfer (Internet), as above - metadata and the data exchanged itself;
- Connection history - when my phone was connected to the [OPERATOR]'s network, when in [OPERATOR'S COUNTRY] or when via roaming;
- Location history - where my phone was located, e.g. which base stations it was connected to or from which country it was connecting or any other information allowing to determine my location more accurately than "Planet Earth";
- History of used devices - IMEI numbers as well as other data collected about my device / devices;
- Any additional information collected about me.