Learn Hardcore Japanese: sensei ha denwa de youken dake saki ni hanasimasu to iimasita. - The teacher said on the phone, “I’ll talk about the matter first.”

先生せんせい　は　電話でんわ　で　用件ようけん　だけ　先さき　に　話はなします　と　言いいました。sensei wa denwa de youken dake saki ni hanashimasu to iimashita.

The teacher said on the phone, “I’ll talk about the matter first.”

Breakdown of sensei wa denwa de youken dake saki ni hanashimasu to iimashita.

Questions & Answers about sensei wa denwa de youken dake saki ni hanashimasu to iimashita.

Why is 先生 followed by は instead of が?

は marks 先生 as the topic: As for the teacher, ...

In a sentence like this, は feels natural because the teacher is already the person being talked about. The sentence is not mainly introducing who said it; it is mainly telling us what the teacher said.

If you used が, it would put more focus on the teacher as the one who said it, as if answering Who said that?

What does 電話で mean here, and why is the particle で used?

電話で means by phone / over the phone.

The particle で often marks the means or method used to do something.
So here:

電話で話します = speak by phone
literally, use the phone as the means of speaking

This is different from 電話に, which would not work here for the same meaning.

What exactly does 用件 mean?