Is it permissible to pray in a mosque where the Fajr prayer is performed a quarter of an hour after the single call to prayer in the town?

It is sunnah (definition) to hasten to perform the prayer after verifying that its time has begun, especially the Fajr prayer, where it is desirable to perform it early, while it is still dark (ghalas), after the true dawn (al-Fajr al-Sadiq) has become clear. Therefore, pray with your mosque's congregation as long as they pray after verifying that the time has begun. If delaying the prayer harms the congregants or causes them hardship, they should be treated gently, and their congregation should be increased by allowing sufficient time between the adhan (definition) and the iqamah (definition). The fact that the mosque performs a single adhan for Fajr does not affect this, even though two adhans are preferred.