A free PBXes account could launder the connection, if you can live with the restrictions (2000 min./mo., 60 min. max per call). If you have an OBi in California, you might use it as a relay, via OBiTalk or another provider.
Who is the SIP provider? Used how (outbound calls to what countries, DID from where, etc.)? Many providers permit connections on alternate ports, or there may be another that better meets your requirements.
How do you know that the ISP is blocking the port (rather than e.g. a router with a problematic SIP ALG)?