Hello
Your best solution is probably to fix the issues with the firewall first, it's more likely that even if you try other solutions you will probably have issues with signalling or media later.Here is a support forum page with a similar firewall configuration like the one you are using: https://supportforums.cisco.com/discussion/10242841/problem-asa5520-asterisk-server-softphone-cant-register
Another possible help is the SX10 does support ICE,STUN and TURN. You can look through the SX10 guide to see where to configure this and there are public free STUN and TURN servers you could use. This will still not help if your firewall is blocking RTP ports.