Most Call Centres have little control over the Incoming Calls therefore the Busy Hour incoming call calculations should be used to size the number of trunks and agents.
Depending on your type of business there may be some mandatory outgoing Call Back Erlangs that need to be made during the Busy Hour therefore I agree you can simply add these outgoing erlangs of traffic to the incoming figure.
‘Call Me Now’ buttons on a website can mean that you have less control of outgoing traffic and may increase your busy hour outgoing call traffic.
However, generally, the Call Centre can take control of when the outgoing calls can be made i.e. outside the Busy Hour when the incoming call volumes are down.