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I've known this for a long time. A quick google confirms that many people think the same.

Basically, that when shutting down a cell phone, the antenna still communicates with the tower. Triangulation will still work, as well as some other stuff.

Only when the battery removed, is a cell phone "really" shut down.

Not that I care but it's interesting to know why!
If possible, please provide primary sources - it's time to shut all the myths around this down!

Update, the following question has been suggested: What is the fastest way to shutdown unrooted Android phone? Obviously, this is semantically NOT equal. The answer there, despite being a very detailed answer, does not provide the reason WHY the baseband processor remains ON.

For a technological feature to exist on perhaps Billions of devices, there must be a need for that... Especially since it drains battery and costs cellular tower bandwidth.

DraxDomax
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    How would anything someone writes here change what you think? If you want a prove you have to do it yourself: Buy a Software Defined radio (SDR) and capture the GSM/UMTS/LTE frequencies used by your phone when it is shut down. – Robert Aug 18 '20 at 09:40
  • I don't understand your comment. I asked for a primary source on "why do cell phones don't really turn off" - do you mean that I am wrong and they actually do? Basically, I would like to know why - like, it costs battery, it costs bandwidth with the tower... If there's a reason, surely there's someone with the technical knowledge to explain, maybe refer to a document... – DraxDomax Aug 18 '20 at 09:54
  • In Android everything radio related is a subsystem that operates inpdependent of the main OS. Usually this subsystem is shut down when you shut down Android. There might be some devices that do not shut down the radio subsystem completely but in my opinion this is more a bug than a "spy feature". The only subsystem that should run when the device is off is the real-time clock. – Robert Aug 18 '20 at 09:58
  • I think your main problem is the definition of "shut down". Shut down is when you press the power button for 2 sec and then select "Shutdown". Your definition of "shut down" seem to be more like "shut down the display (switch the display off)" – Robert Aug 18 '20 at 10:01
  • yeah, I had a feeling that asking this question might make me look like some NSA-paranoid or 5G-hypochondriac... I didn't mean it this way! Have you never heard the same? That cell phones keep talking to the tower when shut down? – DraxDomax Aug 18 '20 at 10:06
  • nah, I mean shut-down as in: press the power button for 2 sec and then select "Shutdown" – DraxDomax Aug 18 '20 at 10:07
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    Does this answer your question? What is the fastest way to shutdown unrooted Android phone? – Irfan Latif Aug 18 '20 at 10:35
  • @IrfanLatif Thanks for the dig-up :) It does confirm that I am not the only guy thinking the baseband processor continues to work even when the phone is shut down! I just wished to know why? What would be a reason to want to keep the baseband processor up despite the user requesting to shut down? – DraxDomax Aug 18 '20 at 10:53
  • why depends on who is asked. i have my own opinion (and i don't believe baseband is necessary for alarm clock) – alecxs Aug 18 '20 at 11:11
  • @alecxs I respectfully disagree. Why does my Diesel car have a DPF? Because it emits toxins and the DPF reduces this pollution. I can refer you to the DPF patent and the reasoning behind it. I find it hard to believe a function like keeping the BB processor alive, has no technical reason... If you were inferring to espionage, that's a reason too, although I think there would be at least a "fake" reason for us to chew on! – DraxDomax Aug 18 '20 at 11:15
  • just because you find it hard to believe doesn't force such reason exist. but good luck finding one anyway – alecxs Aug 18 '20 at 11:26
  • so, you mean that about a million of bugs were fixed in the making of the cell phone as we know it today... And nobody noticed that the radio chip is still on despite the CPU shutdown? Despite draining the battery? Despite being a regular security risk for hackers and deep state to exploit? – DraxDomax Aug 18 '20 at 11:29
  • no, you said. i would say it is willingly designed (and specified) – alecxs Aug 18 '20 at 12:12
  • @alecxs again referring to espionage? I mean, I believe that is very much a possibility. Just in case you were not referring to espionage, would love to hear your reason – DraxDomax Aug 18 '20 at 12:15
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    espionage implies that you are person of interest, i wouldn't use that as synonym for tracking. we do not mark birds with rings because we are interested in this individuals. it's all about the data. i assume commercial interests as origin, political interests as consequences of technical availability. think tanks can predict general possibilities years before a concrete usage case occurs. google maps live traffic or corona warn app are just two examples how that data can be used. telephony carriers cooperate with police since cell phones exist – alecxs Aug 18 '20 at 19:43

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A telephone that is not in contact with the local cell phone tower cannot detect an incoming call. When someone places a call to your phone, you probably want to get notified that your phone is ringing, even if you have hit the power button. The power button turns off the display, but doesn't actually shut down the phone.

Being in communication with the tower uses some battery power. It's quite low, but it isn't zero.

This is true for a smart phone, and also for a dumb phone. A cell phone of 20 years ago also used battery to maintain contact with the tower.

Addendum: When I tap the OFF button on my phone, it goes into Standby mode. The screen goes dark, and I have to enter a password before using it again, mostly. But it still can accept incoming calls, and it alerts me when one comes in. When I go through the POWER OFF procedure, and somebody calls my number, they receive a notice from the phone company that my phone cannot be located. I do not know whether baseband operations are still drawing current.

Walter Mitty
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  • poor wording on my part. By "turned the phone off" I meant "hit the power button". This doesn't reall turn the phone off. Will fix. – Walter Mitty Aug 18 '20 at 22:28
  • question is about completely turned off phone, not about just display/stand-by. baseband is active as soon as sim card and battery is inserted, no matter if phone is on or off (not even sim pin is necessary) – alecxs Aug 19 '20 at 08:52