Yes — Maryland regulation COMAR 10.17.01.21 requires a telephone within the pool enclosure or an adjacent area.
Maryland requires public pools to have a telephone capable of contacting emergency services, located within the pool enclosure or an adjacent area.
Citation: COMAR 10.17.01.21
"Adjacent area" gives some flexibility for pools without room inside the fenced enclosure, but the phone still needs to be close enough for someone to reach it quickly during an emergency.
SouthEast Telephone installs and monitors cellular emergency pool phones nationwide and can help you meet Maryland's requirements.