Yes — Nebraska rule 178 NAC 2 requires the emergency phone to be located within the pool enclosure.
Nebraska requires the emergency telephone to be located within the pool enclosure.
Citation: 178 NAC 2
Unlike states that allow an "adjacent" location, Nebraska's language points to placement inside the fenced pool area itself, so plan for a weatherproof unit mounted on the fence or a nearby post.
SouthEast Telephone installs and monitors cellular emergency pool phones nationwide and can help you meet Nebraska's requirements.