I would like to inform you, based on my experience in hundreds of voip termination and overseas trunk experience, outbound proxy is not a option to solve your problem, proxy definition is different in VoIP it doesn't mean something like doing proxy for voip signalling or media.
it can be just a signalling server that is handling one or more media servers for large scale voip solutions.
they only possible solution to bypass sip and rtp filtering in your country and also many other middle-east countries is sing openvpn, and there is different models to implement that. that's what we're doing on many projects and all working without issue.
Kindly please check my profile for more information about me, and contact me for details.
Regards.