boarding announcement
Used for public address announcements at airports, train stations, or ferry terminals informing passengers that boarding has begun or is about to begin.
When the boarding announcement is made, please head to the gate.
I was careful not to miss the boarding announcement at the airport.
搭乗口 is the boarding gate itself, while 搭乗案内 is the announcement or guidance about boarding.
案内放送 is a general guidance broadcast; 搭乗案内 specifically refers to boarding announcements.
Compound of 搭乗 (とうじょう, 'boarding') and 案内 (あんない, 'guidance, announcement').