The Call Forwarding option on Galaxy S5 and other Android devices lets you forward voice calls (and video calls) to a specific number under different situations, like when the phone is busy, unanswered or unreachable.
To Forward call on Galaxy S5 do as follows:
1. Open Settings app and go to Applications tab.
2. Tap on Call (or you can simple use the search box and look for "Call")
3. Tap More Settings, followed by the Call forwarding option
4. Select whether to forward a Voice call or a Video call
5. Now there are 4 available call forwarding options*, and you just need to tap each option and assign a phone number to it
* Available options are: Always forward, Forward when busy, Forward when unanswered, and Forward when unreachable