Generally, satellite phones are designed to work outdoors, out-of-the box. Accessories can extend their utility to use indoors. To transmit, the satellite phone’s antenna must have an unobstructed direct line-of-sight with the satellites orbiting above Earth, which means the phone/antenna must be outdoors. However, most satellite phones can be used with specialized equipment like an external antenna, which enables use of a satellite phones in a building, vessel or vehicle. A cell phone can be used indoors provided it is in range of coverage, although some materials like thick concrete, steel and iron can obstruct cell signals in some indoor environments.