The phone would show “no coverage” in case it cannot access the nanoBts. In other words, the UL or DL of the BCCH timeslot is “blocked”.
The UL BCCH is the RACH, and if it is “overloaded” then it’s true that the cell will appear as unreachable, and so the MS will read “no nw”.
What doesn’t make sense is why your nano cell would be rach-overloaded… I doubt there are much MS out there… plus, the overload would impact all MS, not only the iPhone.
Have you tried exchanging the sim card from the iphone with a one from a phone which show no sign of misbehaviour ?
The only way to understand the issue is to trace the signalling between nanobts and the iphone. Since very few of us (and clearly not me) have inside knowledge of how the nanoBts and/or the iPhone work and what are their limitations, i doubt you’ll get an answer here. But for sure you need to push IPAccess a bit more. They should be concerned about the issue, and you posting such things on the internet 🙂