Is calling the adhan a sunnah or a wajib?
For the five daily congregational prayers and the Friday prayer it is a sunnah muaqqadah (emphasised Sunnah) which is very close to being wajib (compulsory).
And Allah ﷻ knows the best
Imam Ahmad Rida Khan رحمة الله تعالى عليه
Translation by Dr Musharraf Hussain Al-Azhari Translator of Majestic Quran, www.majesticquran.co.uk