The question asked what is the supplication to say after the mu'addhain has called for the prayer(Azan).

 The question asked what is the supplication to say after the mu'addhain has called for the prayer(Azan).

Alhamdulillah, what is reported from the sunnah is to send salawat to the messenger of Allah salla Allahu alayhi wasalam and said dua saying, 

Allahumma rabba hathihi da'wati taa'mah wa solaatil qaa'imah till the end. 

As reported in Hadith, 

Narrated Jabir bin `Abdullah:

Allah's Messenger (ﷺ) said, "Whoever after listening to the Adhan says, 'Allahumma Rabba hadhihi-dda` watit-tammah, was-salatilqa'imah, ati Muhammadan al-wasilata wal-fadilah, wa b`ath-hu maqaman mahmudan-il-ladhi wa`adtahu' [O Allah! Lord of this perfect call (perfect by not ascribing partners to You) and of the regular prayer which is going to be established, give Muhammad the right of intercession and illustriousness, and resurrect him to the best and the highest place in Paradise that You promised him (of)], then my intercession for him will be allowed on the Day of Resurrection".

Sahih al-Bukhari 614.

Jabir (May Allah be pleased with him) reported:

The Messenger of Allah (ﷺ) said, "He who says upon hearing the Adhan: 'Allahumma Rabba hadhihid-da'wati-ttammati, wassalatil-qa'imati, ati Muhammadanil-wasilata wal-fadhilata, wab'athu maqaman mahmuda nilladhi wa 'adtahu [O Allah, Rubb of this perfect call (Da'wah) and of the established prayer (As-Salat), grant Muhammad the Wasilah and superiority, and raise him up to a praiseworthy position which You have promised him]', it becomes incumbent upon me to intercede for him on the Day of Resurrection." [Al- Bukhari].

Riyad as-Salihin 1039.