Smartphone security tip of the week: Do not use your anonymous SIM card at the same time as a non-anonymous card
If you use an anonymous SIM card, but a second non-anonymous card is inserted in your device at the same time, the anonymous SIM card is no longer anonymous. This also applies to a second phone that you carry with you.
Read more about this topic: https://smartphone-dont-spy.de/en/list/do-not-use-your-anonymous-sim-card-at-the-same-time-as-a-non-anonymous-card