1) Is it obligatory to answer the Adhan when combining and shortening the Maghrib and Isha prayers after the Adhan? 2) What is the interval between two combined prayers while traveling? 3) Is prayer in the mosque obligatory for the traveler?

It is recommended to repeat the Ishā’ prayer with the intention of a voluntary act to attain the reward of congregational prayer. However, repetition is not obligatory because both prayers were performed legitimately. Continuity between the two combined prayers is a condition for the validity of Jamʿ (combining prayers) according to the majority of scholars. A slight separation does not harm, and the length of separation is determined by custom. It is preferable for a traveler to perform the prayer in congregation in the mosque completely, and this takes precedence over shortening the prayer.