What is the ruling of the Shari'ah concerning a man who asks his sister for a girl's number with the intention of marriage, then calls her repeatedly and expresses his feelings to her?

The man erred in his conversation with the woman, for conversations between young men and women—even if for marriage—are a gateway to temptation and corruption, and they must refrain from them. If the man desires marriage, he should approach houses through their proper doors and propose to her guardians. There is no harm for her in accepting him if he is pleasing in his religion and character, after seeking guidance from Allah.