Must I return the meager money I took from my father's property without his knowledge, given that he does not pay my phone bill and I do?

It is not permissible for a child to take from their father's money without his knowledge, unless the father is negligent in providing the obligatory maintenance. In such a case, the child may take what is sufficient for them, according to what is customary, as mentioned in the hadith of Hind bint Utbah. Anything beyond the maintenance remains forbidden in principle, and you must return what you have taken to your father. If you do not know the exact amount, then return what you believe will absolve you, and you are not obligated to inform him of this. The father is not obligated to pay the mobile phone bill unless the phone is used for essential household matters. If the father refuses, it is permissible to pay its bill from your own money.