Does committing forbidden acts over the phone at night invalidate the fast during the day, given that the dawn prayer was performed in the mosque, if this occurred twice?

Speaking with foreign women (non-mahram women) is a sin, whether in Ramadan or at other times. However, it is more gravely sinful in Ramadan. What the fasting person does in this regard does not invalidate his fast, but it diminishes his reward, because the purpose of fasting is to achieve taqwa (piety) towards Allah. The Prophet, peace and blessings be upon him, said: "Whoever does not abandon falsehood in word and action, Allah has no need of his abandoning his food and drink."