Answer
(Fatwa: 1268/1268/M=1431)
If one is offering fajar sunnah then one should complete it. And if it is the four rakah sunnah before Zuhar or Isha and the jamah started, if iqamah was started after starting the sunnah salah, in this case, one should join the jamah and leave the sunnah. And if he had completed one rakah he should complete the second rakah and then join the salah and if he had completed the third rakah he should complete the fourth one and then join the jamah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband