calling subscriber; calling party
Technical term in telecommunications and computing for the party that initiates a call or connection.
When the calling subscriber initiates the call, the receiving terminal rings.
着加入者 is the called subscriber or called party, the recipient of the call, while 発加入者 is the calling party.
Compound of 発 (はつ, 'originate') + 加入者 (かにゅうしゃ, 'subscriber'). The reading はっか is a euphonic change from はつか.