What should I do when I hear the adhan?

When you hear the adhan, repeat its words quietly after the caller, then send blessings on the Prophet ﷺ and make the short dua for him. After that, get ready to pray. Responding is a recommended sunnah with a big reward, not a strict obligation according to most scholars.

1. Repeat after the caller

The Prophet ﷺ said: "When you hear the call, say what the mu'adhdhin says" (Sahih al-Bukhari 611). So after each phrase, you repeat it quietly. There's one exception: when he says Hayya 'ala-s-salah and Hayya 'ala-l-falah ("Come to prayer", "Come to success"), you reply La hawla wa la quwwata illa billah ("There is no power and no strength except with Allah") (Sahih Muslim 385). For the Fajr line "prayer is better than sleep", many scholars say you simply repeat it.

After the shahada lines, you can also add: "I too bear witness... I am pleased with Allah as my Lord, Muhammad as my Messenger and Islam as my religion." The Prophet ﷺ said whoever says this has his sins forgiven (Sahih Muslim 386).

2. Send blessings and make the dua

When the adhan ends, send salawat on the Prophet ﷺ (for example the short "Allahumma salli 'ala Muhammad"), then ask Allah to grant him al-wasilah, a high rank in Paradise (Sahih Muslim 384). The well-known dua is:

Allahumma rabba hadhihi-d-da'wati-t-tammah, wa-s-salati-l-qa'imah, ati Muhammadan al-wasilata wa-l-fadilah, wab'ath-hu maqaman mahmudan alladhi wa'adtah.
"O Allah, Lord of this perfect call and the prayer about to be established, give Muhammad al-wasilah and excellence, and raise him to the praised station You promised him." Whoever says it will receive his intercession (Sahih al-Bukhari 614).

3. Make your own dua

The time between the adhan and the iqamah is one of the times when dua is not rejected (Jami' at-Tirmidhi 212). Use those minutes to ask for anything, in English if you like. See What are the best times to make dua?

4. Head to prayer

The adhan is an invitation, so the natural next step is wudu and prayer. For Friday, the Quran is direct:

إِذَا نُودِىَ لِلصَّلَوٰةِ مِن يَوْمِ ٱلْجُمُعَةِ فَٱسْعَوْا۟ إِلَىٰ ذِكْرِ ٱللَّهِ وَذَرُوا۟ ٱلْبَيْعَ "...When the call to prayer is made on the day of Jumu'ah, then proceed to the remembrance of Allah and leave trade." Quran 62:9

See Is Friday prayer compulsory?

Common situations

In a conversation or on the phone: it's good to pause and respond, but it isn't a sin to carry on.
In the bathroom: stay quiet, and respond afterwards if you wish.
On your period: you can still respond and make the duas.
From an app or TV: many scholars say respond to a live call; for a recording, responding is fine but not expected.

Key points

Repeat each phrase after the caller.
Reply "La hawla wa la quwwata illa billah" to the two "Come to..." lines.
Send salawat and say the wasilah dua at the end.
Use the time before iqamah for your own dua.

Tip: Don't worry about memorizing everything at once. Start by just repeating the words, then add the wasilah dua from a card or your phone until it sticks. See What do Muslims say when they pray?