Hi utm_kid, the ASG already has the ability to do what you want, if I understand your posting correctly. You do not allow your SIP phone to use the http proxy. If you SIP phone is a softphone on a PC, then you have a problem because you can't block 443 without blocking some of your PC access.
You could try changing the http proxy to standard and then configure you SIP phone to use a different port for the secure function.
Being a SIP phone it originates all the traffic, so it must be making offers for other SIP devices to talk to it. If you are using VOIP security then limit the destinations that your SIP phone can register/talk to.
The SIP Phone probably shouldn't be using the http proxy anyway due to the packet lag through the proxy.
Ian M You can use the http proxy classifications to block access to external proxies if you so desire.