What is the ruling on a mobile phone ringing with strange tunes during congregational prayer? Is it forbidden or merely disliked, especially given the existing warnings against it and the fact that some people do not take the matter seriously?

Muslims must strive for khushu' (devotional humility) in prayer by avoiding anything that distracts them. This includes turning off their mobile phones or setting them to silent mode before starting the prayer, because leaving them on causes disturbance and annoyance to the worshippers. If even a patterned garment (khamisah) distracted the Prophet (peace be upon him), then the annoying sounds of mobile phones have an even greater impact in causing disturbance and reducing khushu'. The Prophet (peace be upon him) forbade raising one's voice with the Qur'an if it bothered or distracted the worshippers.

Whoever intentionally leaves their mobile phone ringing, thereby disturbing the worshippers, has committed a disliked act (makrooh) and it may even reach the level of prohibition (hurmah). However, whoever forgets to turn it off incurs no sin, and should immediately silence it, even if already in prayer. This is a minor movement that does not affect the validity of the prayer. If the phone's ringtones include Western or musical melodies, the prohibition is confirmed, because musical instruments (ma'azif) and music are forbidden, based on the Prophet's (peace be upon him) saying: "There will be among my nation people who will deem permissible unlawful sexual intercourse, silk, intoxicants, and musical instruments."