If your PBX doesn’t support SIP check with your manufacturer to see whether a SIP gateway, module or upgrade is available for your existing PBX, that would enable you to use Skype Connect/SIP.
Also, it is highly recommended that a Skype Connect certified device is used, to gaurantee all features and functions work. Following is a link to that lists all Skype Connect certified products:
Please let us know if you have further questions.
Skype Enterprise Support