The Bundesnetzagentur, Germany's regulatory authority, has specific regulations regarding the use of mobile signal boosters[3]. While theoretically the Bundesnetzagentur has laws about what is allowed and what not, it appears that mobile network operators do their own measurements[1]. Field technicians working for Deutsche Telekom can detect when someone uses a signal booster, locate the source, and demand that it be turned off[1]. If the user refuses, the BNetzA may confiscate the equipment and sue for all associated costs[1].
Using signal boosters without authorization can lead to fines and equipment confiscation due to potential interference with cellular networks[3]. Specifically, using these devices can break several laws at once, such as transmitting on protected frequencies without a license and disrupting public telecommunications, which is a criminal offense[1].
Citations:[1] https://www.reddit.com/r/AskAGerman/comments/191j9b2/is_installation_and_using_a_mobile_signal_booster/
[2] https://www.pta.gov.pk/assets/media/ad_cons_paper_10102020.pdf
[3] https://www.mobilesignalguru.com/blog/legal-requirements-and-regulations-for-mobile-signal-boosters/
[4] https://www.weboost.com/blog/regulation-myths-about-cell-signal-boosters
[5] https://mysignalboosters.com/de/faqs/
[6] https://www.bundesnetzagentur.de/SharedDocs/Downloads/DE/Sachgebiete/Telekommunikation/Unternehmen_Institutionen/Anbieterpflichten/OeffentlicheSicherheit/TechnUmsetzung110/Downloads/TR_TKUEV_Ausgabe_8.2_EN.pdf?__blob=publicationFile&v=5
[7] https://www.bundesnetzagentur.de/EN/Areas/Telecommunications/FrequencyManagement/FrequencyAssignment/SatelliteCommunications/SatFu/GNSS/start.html
[8] https://www.federalregister.gov/documents/2013/04/11/2013-07396/signal-booster-rules