Is it permissible for a widowed woman with children to communicate intimately with a married man over the phone, and would her marriage to him be permissible if his wife refuses?

You must sever the sinful relationship immediately. It is impermissible to speak with him on the phone unless it is to arrange marriage matters, provided he is sincere and proposes to your guardian and a valid marriage contract is drawn up. If your soul longs for marriage, then the priority is marriage with the condition of raising the children or leaving them with someone who will undertake that. If there is no need for marriage, then dedicating oneself to raising the children is better and takes precedence. You must ascertain the man's character so that he is not a liar, and the forbidden relationship must be severed until the legitimate marriage contract is concluded.