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.

Hazrat narrates that Qais bin Abi Hazim used to lead us in prayer. One day, he had just completed one rak'ah when the muezzin gave the iqamah for prayer; he left his prayer and stepped forward to lead the prayer.

Hazrat Maymoon says that if you say the Takbeer for Nafl prayer before the Iqamah, then complete the two rak'ahs.

Hazrat Ata says that when you are in the mosque and the iqamah for prayer is called, do not go into ruku (bowing). However, if you have prayed an odd number of rak'ahs, then add one more to it.