I was not aware that the MiFi was blocking anything specific. A list has never been released that could confirm what ports are blocked/allowed by default. It might be worth a call to VZW to see if they could share any more info on the subject.
The override might be somewhere within the Port Forwarding and Port Filtering Tabs of the Admin page. Port Filtering does allow you to enter custom values for allowed/blocked traffic, although it does not display anything as blocked on mine. Play around with those pages and see if you can get it working.
Let us know if you need the steps to access those pages.
Please see the following post for more info about SIP over MiFi devices.
According to Verizon support, the 4510L does not support "custom" port forwarding of UDP packets. It has pre-canned ones in the setup menu. I needed to receieve UDP port 2074 (speak freely app) and it will not pass it. I do know that Verizon does not block this specific UDP port as I am using various cradlepoint routers on Verizon to backhaul UDP audio on that port and it works fine when receieve on my home Comcast Cable Modem.
I have been using VoIP for over ten years on various carriers equipment - I cannot fathom why the latest 4G equipment offering cannot do something my cheesy ten year old router has doing all along. WT F?
Try using a TCP-based VPN (like OpenVPN), which then lets you pass anything. This likely works since the device does not seem to block inbound TCP (most firewalls don't).
THis device is going back as soon as I find one that can do UDP port forwarding.
I am also contacting Novatel Wireless - these are the ****** that made this thing.