A spokesman said the suspect – later identified as Abdoullah C., a Belgian national born in 1985 – had telephoned Hasna Aitboulachen, the cousin of alleged ringleader Abdelhamid Abaaoud, several times after the November 13 attacks and before a police raid five days later in which both Abaaoud and his cousin died.
Abaaoud is believed to have led the team of at least nine gunmen and suicide bombers who killed 130 people and injured hundreds more in coordinated attacks against Paris nightspots.
Belgian authorities have been under pressure to crack down on suspected jihadists in the country after it emerged that several of the Paris attackers, including Abaaoud, lived in the Brussels suburb of Molenbeek.
Police are still looking for Salah Abdeslam, who is believed to have taken part in the Paris shootings and whose brother Brahim blew himself up in the attacks.
Sources close to the investigation say Abdeslam was stopped at police roadblocks three times the day after the attacks as he escaped to Belgium, but was allowed to proceed as he was not yet on the suspect list.
The Paris attacks were claimed by the Islamic State jihadist group.