Ok great, in this case if you can ping them but they do not see any registration attempts coming from you then I would suspect that your requests are not reaching them due to your internet access. This could either be due to the ISP or your own firewall stopping the traffic.
I think it's best to run a capture in this case from one tab, while using a second tab to enable your trunks. The capture should show you the registration attempts and any replies that may or not come back. This is just for verification of course, you will then have to see which element on your network is blocking the attempts.
Did every test pass on your firewall test or did anything fail? (i.e SIP ALG detected)