ANSWER:

If you heard the iqamah, you should have rushed to join the congregation. But if you did not hear it and was not aware of it, your voluntary prayer is valid. 

QUESTION: If the imam in the masjid says the starting takbir of the prayer and I’m praying the sunnah prayer but I’m in home and also I didn’t hear the imam, would that sunnah prayer be valid (for example in case of Fajr where you pray sunnah and go and see one rakah has gone) because after the takbir no other prayer is valid.