Allah's Messenger (ﷺ) said, "Whenever you hear the Adhaan, say what the Mu'adh-dhin is saying. The Prophet (ﷺ) called both Adhaan and Iqamah as Adhaan. [१] Here is to be mentioned that, to reply 'Qaadaqaamatissalah' as 'Aqaamaha allahy wa aadaamahaa' - the hadith is weak (Da’eef) [२] So, for replying the iqaamah, the musalli should reply by using same words as the ad’aan.

Reference: [१] बुख़ारी: ६११ [२] ज़ईफ़ (अल्बानी)। अबू दाऊद: ५२८