Can I use SIP outbound proxy to bypass NAT?

Guys,
I want to make voip call from LAN/softphone to PSTN across ISP network, which block all voip ports. Do you think below network setup can accomplish my goal? Can I bypass NAT router/ ISP firewall by putting SIP Proxy between ISP and SIPserver to perform NAT traversal?

[PSTN]—-(FXOgateway)—-{Router}—-(ISP)—-{Router}—-(SIPserver)—-[LAN]

Thanks for all your inputs.

hkl