Where does the Amidah "officially" start?

Question

In most of the Birnbaum Siddurim editions, before Musaph, he says that the verse כי שם ה' אקרא.. is not recited during Shacharit or Ma'ariv as this would constitute an interruption between גאולה and תפילה.

(Briefly explained - During the morning and evening prayers, there is a theme of redemption said in the various blessings prior to the Amidah . Talmud Brachot mentions that these two items should be together without interruption.)

To me, this implies that the next verse, ה' שפתי תפתח is not considered an interruption, as it is included in all the prayers. Does this mean that the Amidah "officially" begins with that verse rather than the start of an actual Bracha (staring the Baruch of avot)?

Not, that I'm necessarily relying specifically on Birnbaum's explanation, but, I'm trying to understand why he states that one verse would be considered an interruption and the other not.

Answer 1

This is addressed in Berachot 4b:

דאי לא תימא הכי שחרית היכי מצי סמיך והא אמר רבי יוחנן בתחלה אומר ה׳ שפתי תפתח ולבסוף הוא אומר יהיו לרצון אמרי פי אלא התם כיון דתקינו רבנן למימר ה׳ שפתי תפתח כתפלה אריכתא דמיא הכא נמי כיון דתקינו רבנן למימר השכיבנו כגאולה אריכתא דמיא

For, if you do not admit that, how can he join in the morning, seeing that R. Johanan says: In the beginning [of the Tefillah] one has to say: O Lord, open Thou my lips [etc.], and at the end one has to say: Let the words of my mouth be acceptable? [The only explanation] there [is that] since the Rabbis ordained that O Lord, open Thou my lips should be said, it is like a long Tefillah. Here, too, since the Rabbis ordained that ‘Let us rest’ should be said, it is like a long Ge'ullah. (Soncino translation)