This document describes how much will the cost be to forward a call from Yahoo! Messenger to another phone.

You'll incur Phone-Out charges when Messenger phone calls are forwarded, so you'll need a Phone Out account to use Call Forwarding. Rates are the same as Phone Out rates. Calls forwarded to landline phones won't incur an additional charge, but calls forwarded to mobile numbers may count as minutes used, depending on the plan you've purchased from your mobile carrier.