Hazrat Abdullah bin Utbah says that one should break his own prayer and join their congregation.

Hazrat Hammad says that I like that he speaks and then joins their group.

Hazrat Sha'bi says that if a person starts an obligatory prayer and the iqamah is called for that same prayer, then he should break his prayer; it will become a voluntary (nafl) prayer for him, and he should join them in the obligatory prayer.

Ibrahim says that if a person comes to the mosque and thinks that the people have already performed the prayer, so he makes the intention to perform the obligatory prayer, and after he has completed two rak'ahs, the iqamah for the prayer is given. Now he should join the imam in his prayer; when the imam has performed two rak'ahs (making his total four), he should say salam and remain with the imam for the rest of the prayer, and make the last two rak'ahs with the imam as nafl (supererogatory) prayers.

Hazrat Hammad also says the same.

Hazrat Sha'bi says that if a person is offering a voluntary (nafl) prayer and hears the call for iqamah during the prayer, he should break his prayer. Ibrahim says that he should add another rak'ah to the one he has performed and not break it.

4937 - And Ibrahim said: (He adds) (1) (to it) (2) another (and does not) (3) break.

Ibrahim used to say that if anything remains from your prayer, then complete it. Hazrat Saeed bin Jubair says that one should break his prayer.

Ibrahim says that when someone starts a voluntary prayer and the call for the obligatory prayer (iqamah) is given, he should complete the voluntary prayer.

Hazrat Abu Ishaq says that I was standing with Hazrat Abdullah bin Ma’qil. He was performing prayer and reciting Surah An-Nur in the prayer. Meanwhile, the muezzin pronounced the iqamah, so he performed ruku and sujood, then sat and recited the tashahhud. Then he stood with the imam and started from where he had reached.