Learn Polish: Słyszysz? Telefon dzwoni. - Do you hear? The phone is ringing.

It comes from the verb słyszeć (to hear) and belongs to the -ę / -ysz conjugation group you are learning right now. For the "you" (singular) form, we drop the ending from the dictionary form and add -ysz.

What conjugation group is dzwoni in?

The dictionary form is dzwonić (to ring, to call), which belongs to the -ę / -isz group. Because the subject telefon is a "he/she/it" (third-person singular) noun, we use the matching ending for this group, which is just -i.

The English translation says "is ringing". Where is the word for "is" in Polish?

Polish doesn't have a separate continuous tense like English does with "is ringing". A single Polish present tense verb like dzwoni means both "rings" and "is ringing". The context tells us it is happening right now!

Why is it just Słyszysz? instead of Czy ty słyszysz?

The verb ending -ysz already tells us exactly who is doing the action ("you"), so pronouns like ty are usually dropped unless you need extra emphasis. You also don't strictly need the question word czy in casual speech; simply raising the pitch of your voice at the end of Słyszysz is enough to make it a question.