Abstract: In electronic communication systems, negotiations are performed between two pieces of apparatus (e.g. modems) in order to ensure technical compatibility during subsequent data transmission. Instead of performing security checks in the known manner after the negotiations have been completed (e.g. sending an access code or identity code), the invention teaches that the negotiations themselves should be customized to include security checks. For example, during negotiations between a call modem and an answer modem, the call modem includes a "first indication" in the negotiations. The answer modem detects the "first indication" and permits the call modem to have access to restricted access data during data transmission after the negotiations have been completed. The "first indication" may comprise a negotiation segment having a precise length and/or containing an embedded code.