Verizon 4G LTE with Pantech UML 290 NAT breaks VoIP with mobile routers.

Question

LONG story short.  I run a training class once a month (8 road, 4 home) called FreedomVoice University where we train our dealer network on our hosted VoIP phone system called FreedomIQ.  I'm looking to provide my own Internet solution for these classes since most hotels have inadequate (or non-working) wifi and internet.  They typically also lack a wired connection, something that would allow us to connect Polycom (RJ45 connection) VoIP phones for training purposes.  My idea was to use a Verizon 4G USB device that interfaces with a Cradlepoint CBR400 router to provide Internet to a switch that connects multiple VoIP phones.  It turns out that this scenario is impossible to use with hosted VoIP because Verizon has the 4G connection NAT'd.  When you use another router to connect the phones, now a double NAT scenario exists (premise based router providing NAT and 4G connection NAT'd) and that breaks nearly all flavors of hosted VoIP.  Verizon refuses to disable the NAT on their end, citing "security reasons" but I think it's more of a "we don't want you using VoIP instead of our minutes" reason.  Anyway, their 4G LTE service is completely useless to me in this scenario.  Verizon suggested that I turn off NAT on my end.  Apparently they are clueless as to the purpose of NAT.  Multiple devices need unobstructed internet access here guys!!!

Answer 1

The firmware is up to date on the Cradlepoint CBR400.  It's definitely not in the CBR400, i've already pursued that.  Even though NAT is required on my end for multiple phones, disabling NAT by setting the CBR400 to IP passthrough mode to allow the phone to pickup the DHCP address from Verizon didn't work either.  This works on a SIP softphone (Bria by Counterpath) and a laptop but not directly to a Polycom phone.  Very odd scenario... The traffic coming from the Bria app on the laptop probably looks slightly different (no QoS tagging for one) than the traffic directly from the Polycom phone, which leads me to believe Verizon is doing some specialized handling of SIP traffic when they see it (probably SIP ALG of some sort) which is breaking hosted VoIP.  For premise based VoIP it might work but a hosted VoIP scenario is different.  Since the server is in the cloud, if Verizon is manipulating those packet headers before it reaches the server (re-writing a real IP with a LAN IP for example), the hosted server won't know where the traffic came from to be able to communicate back and establish the call (SIP signal or audio session).  This is very common (SIP ALG) in today's routing devices.  This is a whole separate scenario to double NAT which I believe they are also doing.  This would explain why the SIP softphone Bria won't work with Verizon 4G LTE behind the NAT of the Cradlepoint CBR400.
Bottom line is that over the past week i've tested over a dozen scenarios with the 4G LTE and the Cradlepoint CBR400 and determined Verizon is breaking it.  AT&T iPhone tether actually shows some promise except that the Cradlepoint CBR400 isn't compatible with it via USB.  I'd be very curious to know if a compatible AT&T USB device would suffer from the same issues.  In light of my iPhone tether testing i'd have to say no.