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.