Although I tend to avoid commenting on my own posts (a sign of narcisim, I’m sure!), I did want to log the general problem with the Open C’s proximity sensing, as noted here:

PC Mag’s Open C Review
TechRadar’s ZTE Open C Review
Modaco forum chat regarding missing Open C proximity sensor
Mozilla Support forum
Bugzilla (Mozilla) issue tracker

Not directly related, but more info on proximity sensor with ZTE Open (the original): https://groups.google.com/forum/#!msg/mozilla.dev.b2g/Ly1LWN6SRJ0/8dq1FnBj6NEJ