Is it obligatory to repeat the phrases of the adhan for prayers when it is made?

What is recommended, but not obligatory, is to repeat the phrases which constitute the adhan [call to prayer] after the muezzin [the person giving the call to prayer]. Thus, we should repeat the phrases ‘Allahu Akbar’ [God is supreme] or ‘la ilaha illa Allah’ [there is no deity other than God] after the muezzin. However, we should not repeat ‘hayya ‘ala al-salat’ [come to the prayer] and ‘hayya ‘ala al-falah’ [come to success] but rather say ‘la hawla wala qowwa illa billah’ [there is no power or strength except through God]. Thus our answer means, 'Yes, we would like to come, but we pray God to grant us the power to do so, as all power is through Him.' It simply signifies that our ability is through God the Almighty.

At the call to dawn prayer, the muezzin adds the phrase ‘al-salat khayrun min al-nawm’ [prayer is better than sleep]. We should not repeat this phrase but say ‘sadaqta wa wa’adta wa barrat’ [you have spoken the truth, made a promise and fulfilled it].

Is it not proper to repeat the phrases of the adhan when one is in the bathroom as it consists of phrases that mention God's name [since present day bathrooms generally include toilets].

There is no obligation to do anything when we hear the adhan; it is only recommended to do the above. Our response to the adhan should be confirmed by attending the prayer as soon as possible. Needless to say, the best we can do is to offer the prayer with the congregation in a nearby mosque. But if one is unable to do so, one should offer the prayer as soon as possible.
And God the Almighty knows best.