Is it forbidden to masturbate while a married couple is talking on the phone, given their physical distance from each other, and what is the punishment for that, and does it necessitate ghusl if a finger is inserted?

Masturbation by hand or similar means is forbidden and a reprehensible act. It is even more reprehensible for a married person. It is not permissible for you or your husband to masturbate with the hand or finger, and there is no specific punishment for it in Islamic law. It is a sin among sins; if one commits it and does not repent, then he is under the will of Allah. As for one who repents, Allah will accept his repentance. If semen is discharged during conversation between you without the use of hands or other means, this is permissible, and the discharge of semen due to desire necessitates ghusl (ritual bath). Both of you must avoid forbidden masturbation, and the husband should not be absent for too long. If this is not possible, then each of you must be patient, seek Allah's help, and take measures to maintain chastity.