The Bible nowhere mentions cyber-sex (cybersex) or phone sex, obviously, because "cyber-anything" and "phone-anything" were not possible in Bible times. The answer to this question depends somewhat on whether the people involved are married to each other. Within marriage, cyber-sex / phone sex would fall under the "mutual consent" principle of 1 Corinthians 7:5. For more information, please see our article on what is sexually allowed in a marriage.
Outside of marriage, the Word of God gives us some principles that would definitely apply to cyber-sex / phone sex. Philippians 4:8 tells us, “Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things.” Further, there are many Scriptures which indicate that sex outside of marriage is a sin (Acts 15:20; 1 Corinthians 5:1; 6:13,18; 7:2; 10:8; 2 Corinthians 12:21; Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3; Jude 7). Jesus Himself taught us that to desire something that is sinful is also sinful: “You have heard that it was said, 'Do not commit adultery.' But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart” (Matthew 5:27-28). Proverbs 23:7 says, “For as he thinks within himself, so he is.”
Cyber-sex and phone sex are, in essence, desiring something that is sinful (fornication or adultery). Cyber-sex and phone sex are fantasizing about that which is immoral and impure. In no sense could cyber-sex or phone sex be considered noble, right, pure, lovely, admirable, excellent, or praiseworthy. Cyber-sex and phone sex are virtual adultery. They are fantasizing about a person lustfully and encouraging another person into immoral lust. They lead a person into the trap of “ever-increasing wickedness” (Romans 6:19). A person who is immoral in his/her mind and desires will eventually become immoral in his/her actions. Yes, outside of marriage, cyber-sex and phone sex are sinful.