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.
nfon is an IP-Centrex provider for enterprise customers. As the German market leader, nfon offers its technology also as a white label solution to other providers.