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.
Hosted telephony supplier FLR Spectron specialise in business VOIP (voice over IP - voice over internet protocol) and SIP (session initiation protocol) using the new BT Hosted solution. Tel 0845 899 9900