The reason you are getting a 403 error is because you’re not sending the call from a Skype Connect account.
Skype Connect uses the From header, which should contain <SIPusername>@sip.skype.com to resolve which account is making the call, for example:
From: <sip:firstname.lastname@example.org >
where 9905100000xxxx is the SIP username.
If the From header does not contain the SIP username and sip.skype.com domain, we cannot find your account and will forbid the session to take place, that is the call will fail or be rejected.