If the Iqamah for the obligatory Salah is announced while one is offering the last Ruku` or Sujud (prostration), one should complete it and then join the congregational Salah, and there is no need to interrupt it, as Allah (Exalted be He) says:
...and render not vain your deeds.
However, if Iqamah is announced while one is at the beginning of the Sunnah (supererogatory) Prayer or in the second Rak`ah before offering Ruku`, then one should stop, as the Prophet (peace be upon him) said:
When the prayer commences, then there is no prayer (valid), except the obligatory prayer.
Muslim. The minimum limit of Salah is one Rak`ah (unit of Prayer).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.