On the US southern border, some of the arriving people are detained currently. It is irrelevant here why and how that happens.
Cell phones are confiscated when a person found to be an illegal immigrant is detained on the border. In this case, there is some indication that the person has done, or is expected to do something illegal.
He/she is in investigative custody/pre-trial confinement. And it is normal the person may not be allowed to have a cellphone or any other communication device.
(The prison system in Germany is quite different, so let me know if it's wrong)
As far as I know, an asylum seeker is detained in a similar way.
But it is legal to seek asylum.
In this case, all the above does not apply.
So what is the legal base to confiscate personal property at the time of detention? (In this example a cellphone)?
The practical effect of it seems to be that some asylum seekers are detained in conditions that are possibly unacceptable, but can not document the conditions because they do not have access to their smartphone.