Wiktionary
n. 1 (context nautical English) The first port that a vessel calls in at after the start of a voyage 2 (context idiomatic English) The first place to go to start a process.
Usage examples of "first port of call".
By the time we were over the bright blue waters of the Mediterranean and heading for Jerusalem, our first port of call, I had started to get the feel of The Rover.
You must have realized that from now until you sail you will be followed at every step, but you may ask yourself why we do not fear that later, at the first port of call, you will be tempted to escape and hide.
She did not seem interested in us, and was not to be seen in the harbor at our first port of call.
His first port of call was Paros where he fought with the sons of Minos.
Trafford had made Slithila his first port of call after lifting from the planet of the hallucinogens, for only one reason: it was the nearest world with a regular service of interstellar passenger liners.