Is it permissible to interrupt a Sunnah prayer when the muezzin calls to prayer, or must one complete it?

If a worshipper begins a supererogatory (nafila) prayer and then the congregational prayer is called, he should continue his nafila prayer, making it brief, and not قطع (cut it short/terminate it); this is based on Allah's saying: "And do not invalidate your deeds" (Quran 47:33). However, if he fears missing the virtue of the congregation, he should terminate the nafila prayer and join the Imam, because praying with the congregation is obligatory, and an obligation takes precedence over a voluntary act. The prohibition mentioned in the hadith, "When the prayer is called, there is no prayer except the obligatory one," specifically applies to initiating a nafila prayer after the congregational prayer has been called.