Elon Musk is pushing into the cell phone carrier business, and it's bad news for Verizon and AT&T.

• Musk's company agreed to buy wireless spectrum for billions of dollars, a deal needed to launch Starlink-based cell service.
• Verizon and AT&T stocks fell sharply within hours of the announcement as investors realized a new competitor was coming.
• Starlink's satellite network could cover dead zones where traditional carriers lose signal, a direct threat to their service advantage.
• Musk is also facing pushback abroad, including resistance from a powerful Indian billionaire who doesn't want Starlink operating in India.

Outlook: Expect Verizon and AT&T to face growing competitive pressure as Musk's satellite-based wireless plans move toward launch.