What is the ruling regarding Dhuhr prayer if the muezzin gives the call to 'Asr prayer while I am in the second rak'ah?

Scholars have differed regarding one who catches less than a full rak'ah of a prayer before its time expires: The Hanafis and Hanbalis held that whoever makes the takbirat al-ihram (opening takbir) before the time expires has caught the prayer as ada' (performance within its prescribed time). As for the Malikis and Shafi'is, they held that one does not catch the time unless a complete rak'ah is performed. This latter view is the most preponderant, based on the Prophet's (peace be upon him) saying: "Whoever catches a rak'ah of the prayer has caught the prayer." As for one who is asleep, they are excused, and must perform the prayer immediately upon waking up, based on the Prophet's (peace be upon him) saying: "Whoever forgets a prayer or sleeps through it, its expiation is to pray it when he remembers it."