If a person hears Kaddish or Kedusha immediately after he recited the introductory verse to the Amida – "Ado-nai Sefatai Tiftah U’fi Yagid Tehilatecha" – but before he began reciting the Amida, should he answer to the Kaddish or Kedusha? During the Amida prayer, of course, it is forbidden to interrupt for any purpose, even to answer to Kaddish or Kedusha. Thus, for example, if a person hears Kaddish or Kedusha as he recites the Beracha of "Tishkon Be’toch Yerushalayim," he may not respond; at most, he remains silent. In fact, even after one concludes the final Beracha of the Amida – "Ha’mevarech Et Amo Yisrael Ba’shalom" – he may not respond to Kaddish or Kedusha, unless he has recited the verse, "Yiheyu Le’rason Imreh Fi." After one has recited "Yiheyu Le’rason," various rules apply regarding permissible interruptions. But it is clear that during the Amida prayer itself, one may not interrupt for any purpose, even to respond to Kaddish or Kedusha.
The question arises as to whether this applies already once a person began reciting the introductory verse of "Ado-nai Sefatai Tiftah." Since this verse is not, technically, part of the Amida, and merely introduces the Amida, perhaps it is permissible to interrupt for Kaddish or Kedusha, provided that one has not yet begun reciting the actual Amida prayer ("Baruch Ata Hashem Elokenu…").
The Halacha in this case depends on the prayer which one is reciting. If this occurs during Shaharit or Arbit, then one may not interrupt to respond to Kaddish or Kedusha. Halacha requires beginning the Amida prayer at Shaharit immediately after the Beracha of "Ga’al Yisrael," and beginning the Amida prayer at Arbit immediately after the Beracha of "Shomer Amo Yisrael La’ad." Therefore, even though "Ado-nai Sefatai" is not technically part of the Amida, one may not interrupt to respond to Kaddish or Kedusha.
However, if this occurs during Minha, then, according to the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Vayakhel (10), one may respond to Kaddish or Kedusha after reciting "Ado-nai Sefatai Tiftah," since he has not yet begun the actual Amida. The Ben Ish Hai adds that after one finishes answering to Kaddish or Kedusha, he should then repeat the verse of "Ado-nai Sefatai Tiftah" before beginning the Amida, as this verse must be recited immediately before the recitation of the Amida.
In this context the Ben Ish Hai addresses the case of a person who heard Kaddish or Kedusha immediately after reciting the first three words of the Amida – "Baruch Ata Hashem." In such a case, it is questionable whether the person should recite the words "Lamedeni Hukecha," so that he will have recited the complete verse of "Baruch Ata Hashem Lamedeni Hukecha" (Tehillim 119:12). Perhaps, he can then be considered as not having begun the Amida, so he can respond to Kaddish or Kedusha. The Ben Ish Hai writes that he is uncertain whether one should recite "Lamedeni Hukacha" and then answer to Kaddish or Kedusha in such a case, and he leaves this question unresolved.
Summary: If a person recites the verse of "Ado-nai Sefatai Tiftah U’fi Yagid Tehilatecha" to begin the Amida prayer at Minha, and before he recites "Baruch Ata" he hears Kaddish or Kedusha, he should respond to Kaddish or Kedusha, and then begin again ""Ado-nai Sefatai Tiftah…" If this occurs at Shaharit or Arbit, he should not respond to Kaddish or Kedusha.
