I don't think that a static IP will be causing this issue because the sip connection uses VoIP/voice wan profile for passage and not internet wan profile. That is why we have added the VoIP gateway while creating a static route so that we can ping the sip server through that particular gateway only.I Also Bought A Public IP. Will This be causing issue, Pinging SIP says Request Timed Out
This is why we have to remove interface binding.Other than the subnet, the issue might be because of binding the interfaces. Since internet is bound to every other interface, VoIP is restricted on those ports.