When call forwarding is activated on any direct number or SIP login, all incoming calls to that number will be directed to your phone (to the number you specify when setting up forwarding).
For example, say you’re in New York, and have only your cell phone with you. You want to talk to clients in Moscow and London, without using your cell phone plan. The ideal solution - you purchase virtual numbers in Moscow and London, and set the call forwarding to your number in New York. Your clients will call a local (to them) number, and you’ll get the calls on your mobile device.