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.
Orpheum offers its iVoice (SIP trunking) and hPBX (Hosted/Virtual PBX) services exclusively to businesses in the USA and Canada. Both services start at $25 per month.