ISM1600H4 is New 900Mhz Land Phone Model wireless Intercom system. It is expandable up to 1000 lines any time. Range up to 1000m in open area. External antenna you can fix out side of Window or Building. Battery backup facility also available on request. 900Mhz Mobile Models ISM1600H3 and 900Mhz Land Phone model ISM1600H4  are compatible with each other