I prayed Maghrib on time, and I did not know that the 'Isha' call to prayer had already been made.

If a Muslim prays Maghrib, believing it to be within its time, and then it becomes clear that it was after the 'Isha' adhan, their prayer is valid and they are not required to make it up. This is because the intention of performing it (on time) substitutes for the intention of making it up (qada') when one is unaware of the time. It is not permissible to intentionally delay a prayer beyond its prescribed time without a legitimate Shar'i excuse. Whoever does so has committed a grave sin and must repent. The best of deeds is performing prayer at its appointed time.