Debugging IAX connectivity issues


So I am having rather an odd issue that I do not know how to investigate. I installed Asterisk on a raspberry pi and configured it to use IAX, used a softphone to connect and everything is working great on my local network. Then I opened the needed IAX port in my router and tried to test it from another network but it did not work. This strikes me as a NAT issue, but I used a UDP sender program to send a packet to the IAX port, and in the logs chan_iax2 acknowledged receiving a midget packet, this tells me that there are no NAT problems.
My biggest fear now is that my ISP is somehow blocking IAX and SIP traffic, but I can’t think of a way to debug this, or to confirm if my ISP is really blocking it. I am open for your thoughts.