What is the ruling regarding responding to the Adhan heard from others?

Primary text

If the Prophet, peace be upon him, heard the Adhan, he would hold back (from action/departure) until the call was completed. If he heard someone say the initial Takbir ('Allahu Akbar Allahu Akbar'), the Prophet declared it upon the natural disposition (Fitrah). When the caller recited the testimony of faith, the Prophet declared that the caller had been saved from the Fire. This narration by Muslim demonstrates the virtue of affirming the call.