Is cyber sex or phone sex considered an adultery [Zina]....?

As-Salamu wa Rahmatullahi wa Barakatuh.

In the Name of Allah, Most Gracious, Most Merciful.

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger

Islam is a religion of chastity and morality. It aims at building a solid society on bases of purity, modesty, morals, and good manners. By the same token, Islam blocks all ways that lead to evil to the extent that Muslim jurists adopt a juristic principle that reads: What is conductive to haram (unlawful) is itself haram.

As far as the issue of "cyber sex or phone sex" is concerned, we'd like to cite the following fatwa issued by Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America:

"Zina according to Shari`ah is any illicit sexual intercourse. Thus pre-marital sex, extra marital sex and homosexuality are all considered Zina according to Islamic law. Zina is a major sin (Kabirah); It is absolutely forbidden.

Islam does not only forbid illicit sexual intercourse, but also forbids anything that leads to this sin and crime. 'Cyber sex', 'phone sex', etc. are those activities that can lead to Zina. The Prophet (peace and blessings be upon him) said: 'The eyes commit Zina, the hands commit Zina and feet commit Zina and the genitals commit Zina.' (Musnad Ahmad, Hadith no. 4258) In another version, he is reported to have said that 'the genitals confirm or deny it.' (indicating that starring at opposite sex in a lustful way has a spontaneous effect on the genitals and may induce person to commit Zina)

Thus, Muslims should avoid any avenue of Zina. Allah, Most High, says, 'Do not even come close to Zina. It is a shameful deed and an evil way.' (Al-Isra': 32) So Allah, in the Qur’an, not only prohibits Zina, but He also prohibits even coming close to it."
JazakAllah Khair.......!!!