Many VOIP carriers listen on other ports besides 5060. (Typical ranges to listen on are like 5060-5080.) I had periodic problems when I was with Comcast with 5060 getting blocked; I'd need to change my VOIP ATA to 5060, 5062, etc., periodically.
I never figured out why the port would get blocked, although I assume it was some sort of spam filtering or abuse prevention. It didn't seem to be an attempt to block SIP outright, since it was easy to get around; if your ISP is honestly trying to prevent you from using VOIP, then the workaround might not be quite so trivial.
But I would "try before you pry" and just go up to 5061 or something else in the listening range before you get started with forwarding through a remote server.