How are messages being sent without internet in CJP protest? Is this technology being used?

When 4G and 5G internet was shut down in areas like Connaught Place and Jantar Mantar of Central Delhi due to performance, the question arose as to why this is being done. However, even after the internet is down, messages are being sent and each other is being contacted, now how is this happening without internet? This is becoming a big question.