The Securities And Exchange Board of India (Sebi) has directed its regulated and registered entities to use the ‘1600’ phone number series for service and transactional voice calls to existing customers. This will help investors easily identify calls from Sebi-regulated firms and avoid "unscrupulous entities" using regular 10-digit numbers. The market regulators' decision comes amid increasing cases of telephonic calls and messages by fraudsters posing as Sebi regulated entities.