When you receive an incoming call to your phone number, your other telephones (cell, home phone, car phone) will ring as well and the first one answered will be connected to the caller. Basically the call will be routed to all phone sets, which you will list in simultaneous ring option.
Citel PBXgateways are installed at the central PBX location, and EXTenders are deployed at branch offices, call centers, or at home offices over managed IP connections, DSL, cable, or analog circuits