From the time of amud hashachar it is forbidden for a person to walk to the doorstep of his friend, his father, or his rabbi and greet him with a hello or address him in any other way. If he does, he shows that he is ascribing more importance to that person than to HaKadosh Baruch Hu, for instead of standing in gratitude and in prayer before Hashem, he first goes to greet that person instead (Berachot 14a).
If a person passes someone’s house, and a sense of common courtesy deems it appropriate that he enter to greet him, he is permitted to say “good morning” to him. He may not say “Shalom” since HaKadosh Baruch Hu’s name is “Shalom” and it is not proper to honor a human being with God’s Name before praying (Shulchan Aruch, Orach Chaim 89:2).
If he encounters someone along his way, according to most poskim, he is permitted to say “Shalom” since he did not intend to honor him. There are those who maintain that even in this case, it is better to say “good morning” and not “Shalom” so as to remind himself that he has not yet prayed, and so that he will not be delayed by engaging in a secular conversation before prayer. That is the proper practice (see Mishnah Berurah 89:16). If a friend who already finished praying met him on his way to synagogue and greeted him by saying “Shalom,” he may respond “Shalom” even though he has not prayed (Mishnah Berurah 89:16).
This prohibition only applies when one walks to his friend, father, or rabbi in order to honor him. However, for the sake of a mitzvah it is permitted. Therefore, if, for example, one’s father needs to be accompanied to synagogue, one is permitted to wake up early, greet his father at his door and take him to synagogue. L’chatchilah he should say “good morning” to his father and not “Shalom.”
Similarly, if, in order to honor one’s elderly parents who are about to depart on a journey, one must accompany and help them, and if he prays first he will arrive too late to help, he must first recite Birkot HaShachar, and then escort them to the airport and subsequently pray. (His father will pray on the plane.) The same rule applies when one needs to greet his parents upon their return.
. Kaf HaChaim 89:25 writes in the name of the Acharonim that it is permissible to engage oneself in matters of a mitzvah before praying, as writes the Mishnah Berurah 250:1. Accompanying one’s parents or one’s rabbi to synagogue falls under the category of a mitzvah. Shut B’Tzel HaChochmah 5:70 writes that one is also permitted to accompany them to the airport. He must be careful to recite Birkot HaShachar before that (based on what is brought by the Orchot Chaim and Terumat HaDeshen in the Beit Yosef). (Shevut Yaakov 2:22 is lenient in any case about initiating a greeting to his father or rabbi, since the Torah commands us to honor them. Still, the Pri Megadim forbids it and so does the Mishnah Berurah 89:10).The Mishnah Berurah 89:9 writes that even walking to his friend’s seat in the synagogue is considered greeting him before prayer. Eshel Avraham Butshatash 89:2 tends to be lenient. Bowing is also deemed a salutation (Mishnah Berurah 89:13).Regarding calling someone on the phone, it seems that in times of need, one who needs to make a call is considered like one who passes by a friend’s house, in which case he is permitted to enter while refraining from saying “Shalom,” but it is good that he recites Birkot HaShachar before that. However, if there is no need, calling is forbidden, for then he is considered like one who greets his friend before prayer.