Skype to Skype calls are free of charge, so if someone calls from Skype to Skip2PBX this is certainly a free call.

If your PBX supports the DISA service or auto-attendant features, you can inbound a call to a specific extension simply by dialing the desired extension on the Skype keypad.

Skip2PBX V. 3.x is even supporting a very powerfull routing mechanism wich is capable to route a call based on Caller ID, Called Skype Account and other parameters.