Well yes, what I have always said is that you WILL burn your flash with it. It might be tomorrow or next month but it will happen sooner or later

Hmm well, when the 2nd stage kernel is started there are things that are not fully initialized, so in theory (and that follows, then also in practice) it is possible that some anomalies arise that affect VOIP accounts. Could be something relating to kernel iptables, for example...

Also this might be HW version specific, just like the boot problems that are known to affect rev.1603 devices.

What I'd do is try to find out, first from kernel logging the difference of starting for example the L2fixed kernel directly and via kexec().
When/If the differences are found, next step is to build a kernel that fixes the initialization.