Is it permissible for a husband to divorce his first wife, with whom he has not had children, while having another wife who has kept him from haram, because he is unable to be just in nightly stays?

A husband's discomfort with his wife does not justify engaging in illicit relationships via phone, and he must repent from that. Divorcing the wife is permissible if there is a need for it, such as fearing injustice, but it is better to keep her under his marital bond and to be just between them, especially if she is one of his relatives.