We’ve been wondering.
Is it a must to install carrier FW (For example for Verizon or AT&T) , in order to connect with these networks ?
What will work/not work if we stay with Generic FW ?
The more nuanced answer is that if you populate the CID’s (at+cgdcont) with the right APN’s in the right location then it might work BUT there are some caveats to this.
You will not get network approval and you need this to secure your supply of SIM’s from them.
If you are using a roaming SIM (only feasible on AT&T as Verizon roaming partners have a requirement placed on them to have Vzw approval on applications they supply to) then yes you can use whatever firmware you want.
As part of the attach to the network the unit will declare an SVN (Software Version Number) which starts at 1 for the first approved release and then increments as we approve new versions. I do not know if the networks check this against SIM cards that are using their SIM’s and attach reject ones which are not approved.
There are other parameters that are specified in the operator PRI file that might affect operation that do not go into the generic so this might limit operation and this is the great unknown.