Is repentance or expiation obligatory for one who had phone sex with a foreign woman during the daytime in Ramadan?

The wisdom behind fasting is to achieve taqwa (piety, consciousness of God) by obeying His commands and safeguarding one's limbs from disobeying Him. The ugliness of disobedience intensifies in virtuous times and places, like Ramadan. Whoever engages in phone sex must offer sincere repentance. If semen is discharged, the fast is invalidated, and one must make up the day without expiation. If no discharge occurs, the fast remains valid, but its reward is diminished.