We can't reliably reproduce the problem, it really is random. But I've managed to capture a conversation with WireShark, and when you replay the call you can hear (in this instance) the 8 second delay.
So I've sent that to the installer, and they are passing it on to LG-Ericsson (oops, I slipped) to investigate.
Doesn't even happen consistently with the same customers, so it's not likely that they're on SIP and we're on SIP and there's some incompatibilities between the providers. Also, happening on an internal call would rule that out too (all phones being the same make and model, you'd think they'd have compatible codecs)...