Germany's highest court, based in Karlsruhe in the state of Baden-Württemberg, was to examine 60 separate legal questions regarding the law, which has prompted the biggest class action in the court's history.
The law is meant to make it easier for authorities to gather evidence against criminal suspects including terrorists but opponents claim that it invades the privacy of people who have done nothing wrong.
Introduced in 2008 by the former “grand coalition” government, the law brought Germany into line with a European Union directive of 2006.
It means sensitive personal data detailing phone numbers called and internet sites visited can be kept for every person for six months.
Justice Minister Sabine Leutheusser-Schnarrenberger, of the pro-business liberal Free Democrats (FDP), is among the complainants against the law.
Former Interior Minister Gerhart Baum, also of the FDP and also a complainant, said there was no way the law would stand.
“The stockpiling of data is without doubt in breach of the constitution,” Baum told the daily Hessische Niedersächsische Allgemeine.
“And it affects all citizens, who because of it could become risk factors without having given the least cause for it.”
Even if actual conversations were not recorded, the data was still an extremely powerful breach of privacy, Baum said. It could be used to gather huge amounts of information about who was talking to whom, when and how often.
“From it, for example, you could even tell people's movements from their mobile phone data. It means you could get a movement profile and tell where a person was at the time of the conversation.
A decision is not expected until next spring.