Warning about smartphones and smart devices

When conducting sensitive activities, remember that:

You should not bring your real smartphone or smart devices with you (even turned off). Correlation attacks are possible on the Cell Networks to find which phone “turned off” before your burner phone “turned on”. While this might not work the first time, after a few times, the net will tighten, and you will get compromised. It is better to leave your main smartphone at home online (see this article (Russian, use Google Translate link): https://biboroda.livejournal.com/4894724.html [Google Translate] [Archive.org])
Again, do not take them with you unless it is absolutely necessary. If you really must, you could consider powering it off and removing the battery or, if not possible, the use of a faraday cage bag to store your devices. There are many such faraday “signal blocking” bags available for sale and some of these have been studied for their effectiveness. If you cannot afford such bags, you can probably achieve a “decent result” with one or several sheets of aluminum foil (as shown in the previously linked study).
Warning: consider that sensor data itself can also be reliably used to track you.
Consider leaving your smart devices at home online and doing something (watching YouTube/Netflix or something similar) instead of taking them with you powered off. This will mitigate tracking efforts but also create digital traces that could indicate you were at home.

Lastly, you should also consider this useful sheet from the NSA about Smartphone security: https://web.archive.org/web/20210728204533/https://s3.documentcloud.org/documents/21018353/nsa-mobile-device-best-practices.pdf.

Note: Please do not consider commercial gimmicky all-in devices for anonymity. The only way to achieve proper OPSEC is by doing it yourself. See those examples to see why it is not a clever idea:

AN0M:https://www.theguardian.com/australia-news/2021/sep/11/inside-story-most-daring-surveillance-sting-in-history [Archive.org]
Encrochat:https://en.wikipedia.org/wiki/EncroChat [Wikiless] [Archive.org]
Sky ECC:https://en.wikipedia.org/wiki/Sky_ECC [Wikiless] [Archive.org]

You should never rely on some external commercial service to protect your anonymity.