All devices on the I2C bus are addressed by using the general call address. When the general call address is used, all device on the I2C bus which receive a message with this address should (at least theoretically) confirm the reception by the ACKNOWLDGE ACK bit. In essence, the general call address is one of the addresses reserved by the I2C protocol. These are usually all '0' with the direction bit R/W=0.
In order to enable the recognition of the general call address, the control bit GCEN (I2CCON<15>) which enables general call address has to be set.