Q: Is there a way to dial into a conference bridge via SIP directly? I.e. is there a SIP address assigned to a conference bridge?
A: You can configure your local personal number to forward calls to a conference. E.g. your account has 12345 local number. It may be configured to go to the same conference which already has forwarding through one or more USA or other country DID's.
Any Voovox.com customer can call 12345 via SIP or IAX2. E.g. iax2/usgw.ast.voovox.com/12345 or sip:email@example.com
This is not a public accessible SIP url. SIP phone must be configured with a Voovox AccID/PIN to make such a call.