Take T49G as an example:
Connecting your mobile phone to T49G via Bluetooth. (T49G with built-in Bluetooth)
While there is an incoming call, both mobile phone and T49G will ringing, you can answer it on T49G (or on mobile phone)
On your mobile phone, you can switch this call to T49G and not need to interrupt it.
Hang up the call on mobile phone or T49G, the call will hang up.
T49G can synchronize mobile phone contacts and call the contacts on behalf of your mobile phone.