What is the ruling on interrupting prayer to answer the phone and leaving the mosque, and what is the ruling on instigating a loud argument about this matter in the mosque?

It is not permissible to interrupt the prayer except for a valid reason that permits it, and whoever does so commits a sin. However, if the person was ignorant, they are not sinful if they are among those whose ignorance is excused. In this case, the prayer is invalid.

The Imam's action in condemning it was correct, and there is no harm in raising one's voice in the mosque to condemn a wrongdoing or to teach knowledge.

However, he should have been gentle in his condemnation, especially if the doer was ignorant, for gentleness is the fundamental principle in forbidding evil. The Prophet, peace and blessings be upon him, said: "O Aisha, Allah is gentle and loves gentleness, and He gives for gentleness what He does not give for harshness, and what He does not give for anything else." Moreover, the one who enjoins good and forbids evil must be knowledgeable, gentle, and forbearing.