The external call transfer – is the call transfer to a telephone line other than the company’s telephone line. The internal call transfer – the transfer of an incoming call to another addressee registered on the same telephone line.
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.