Does Sexting Break Your Fast?
Question

Is repentance or expiation required of one who engages in phone sex with a non-Mahram woman during the day in Ramadan?
Praise be to Allah.Purpose of fasting 
The purpose of fasting is not simply to refrain from eating, drinking and fulfilling desires; rather the purpose is to attain piety and mindfulness of Allah, may He be Exalted, by doing what Allah has enjoined and guarding one’s physical faculties from committing acts of disobedience to Allah, may He be Exalted. Allah, may He be Glorified, says (interpretation of the meaning):
“O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous.” [Al-Baqarah 2:183].
The Prophet (blessings and peace of Allah be upon him) said: “Whoever does not give up false speech and acting upon it, and ignorance, Allah has no need of him giving up his food and drink.” (Narrated by Al-Bukhari (6057)
He (blessings and peace of Allah be upon him) also said: “There may be a fasting person who gets nothing more from his fast than hunger and thirst, and there may be a person who prays Qiyam and all he gets from his Qiyam is a sleepless night.” (Narrated by Ibn Majah, 1690; classed as authentic by Al-Albani in Sahih Ibn Majah)
`Umar ibn Al-Khattab and `Ali ibn Abu Talib (may Allah be pleased with them both) said: “Fasting is not only abstaining from food and drink; rather it is also abstaining from lying, falsehood and idle pursuits .” 
The one who is fasting should pay attention to that, and make his fast a means of self-discipline, controlling his desires and attaining piety and mindfulness of Allah, may He be Exalted.
Sins are more abhorrent in Ramadan 
The scholars stated that sin increases in abhorrence if it is committed at a time of virtue or in a place of virtue.
All sins are abhorrent, but they are even more so in Ramadan.
If a person is fasting only for the sake of Allah during the day in Ramadan, refraining from fulfilling his legitimate desires, then his fast is indeed for the sake of Allah: “He gives up his food, drink and desire for My sake. Fasting is for Me, and I shall reward for it, and a good deed will be rewarded tenfold.” (Narrated by Al-Bukhari and Muslim); this version was narrated by Al-Bukhari (1894). So how about the one who is committing filthy actions with a non-Mahram woman??
Hence what the one who has done anything of what is mentioned in the question must do now is hasten to repent sincerely to Allah, may He be Exalted, by regretting what he has done, refraining from it, and resolving never to go back to this sin in the future. He should also cut off all ties with this woman who is forbidden for him, and others like her.
By doing that, his repentance will be sincere and there is the hope that Allah, may He be Exalted, will accept it and will forgive him his sins.
Does sexting break your fast?
If sexting led to emission of semen , then the fast is spoiled and he must make up that day. As for expiation, it is not required, because the correct scholarly view is that expiation is only required in the case of intercourse [during the day in Ramadan].
If sexting did not lead to emission of semen , then his fast is valid, and it is sufficient for this person to repent to Allah, may He be Exalted.
What is meant by the view that his fast is valid is that he does not have to make up that day again. But his fast is one thing, and what Allah wants for His slaves from the fast, and what He loves from them, is something else altogether.
Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever does not give up false speech and acting upon it, Allah has no need of him giving up his food and drink.” (Narrated by Al-Bukhari, 1804)
Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “There may be a fasting person who gets nothing more from his fast than hunger and thirst, and there may be a person who prays Qiyam and all he gets from his qiyam is a sleepless night.” (Narrated by Ahmad, 8693; classed as authentic by Ibn Hibban, 8/257, and by Al-Albani in Sahih At-Targhib, 1/262)
And Allah knows best.