What does one do when the obligatory prayer is called for while he is praying a voluntary prayer?

Question:
The one who is praying a voluntary prayer and has completed the first unit, and then the prayer is called for?

Answer:
He breaks it off and attends to the obligatory prayer.
Q: ... the recommended practice of Fajr?
A: He breaks it off and prays it after Fajr, he prays it after the prayer, or after the sun rises. If the call is made, he breaks it off, unless he is in the last part and has already bowed the second bow, in which case there is no harm.
Q: He prays it after the prayer?
A: If he wishes, he prays it after the prayer, and if he wishes, he prays it after the sun has risen. He is given the choice, and after the sun has risen is better if it is feasible.