You cannot call a sip based phone from Skype (directly). The protocol used by the two systems is different and they cannot talk to eachother. It seems that SkypeIN numbers are forwarded to your Skype number so again they will not work to a SIP device. You may be able to forward the call to a PSTN number. Check this site for more info
Voip Solution One more thing there is one new Sipprovider who is providing local number that can be used to call an online Skype users from a normal phone.