I know nothing about SPA..., but for a FXO device to hanup, it depneds:
1. a busy tone return from remote
2. detect a certain period of slience period.
If you call from pstn, when you hand up, your side will return a busy tone to FXO, so FXO will on-hook.
If SIP hang up first, it seems that Axon doesn't return busy tone to FXO such that SPA still off-hook.