This line is the first direct address (or apostrophe) that lets us know that this entire poem is written to the speaker's father. Throughout the poem, the speaker returns to this "you," saying "Daddy," just to remind us that, hey, we're talking to somebody here! This entire poem is a communication to the speaker's father, who, as the poem later tells us, was not easy to talk to even when he was alive.
I used to pray to recover you. (line 14)
Prayer is, after all, a way to communicate with God, or with those whom we've lost. Here, the speaker has just started to experiment with ways to communicate with her dead father. We see her struggling with this through the whole poem.
In the German tongue, in the Polish town Scraped flat by the roller Of wars, wars, wars. (lines 15-18)
Now the speaker is giving us a whole new language to work with – German, that she probably picked up from her German father. Throughout the poem, the father is shown as a German – a Nazi, to be specific. So it's appropriate that the speaker gets caught up in the German language. Just like the Polish town gets destroyed by wars, the speaker is destroyed by the German language.
I never could talk to you. The tongue stuck in my jaw.
It stuck in a barb wire snare. Ich, ich, ich, ich, I could hardly speak. (lines 24-28)
Now we see a more vivid description of how German is an obstacle to the speaker's communication with her father. She tells us how she can't speak to her father, and then we hear her stammering, saying "Ich, ich, ich, ich" – "I," in German. So not only is she struggling to talk, but she's having a hard time speaking the word "I" – a word essential to her identity, which has been overpowered by her father.
And the language obscene
An engine, an engine Chuffing me off like a Jew. A Jew to Dachau, Auschwitz, Belsen. I began to talk like a Jew. I think I may well be a Jew. (lines 30-35)
Now that we've heard the speaker struggle to communicate in German, we get a glimpse of the German language's power over her – it's no longer a language, but a train, taking her away to a World War II concentration camp. German has overpowered her so much she even begins to speak like the victims of the German Nazis, like a Jew.
The black telephone's off at the root, The voices just can't worm through. (lines 69-70)
Here, the struggle to communicate is over – the speaker is taking back the power. She's disconnected the "black telephone" which connects her with her dead father. She's building up to the end of the poem here, telling her father, for the last time, that she is through.