When you take the phone line off hook maybe there is a little "bounce" in the electrical circuitry and you are getting a false hook flash.
The OBi Admin Guide says the minimum hook flash time is 100 ms, but that is not truthful. My OBi's show the minimum hook flash time setting of 70 ms. What happens if you increase the minimum hook flash time to 100 or 125 ms?
Physical Interfaces --> PHONE Port --> Timers --> HookFlashTimeMin
Hook Flash is a quick transition of the phone's hook switch from Off-Hook state to On-Hook state, and back to Off-Hook state.
This parameter specifies the lower time limit in
milliseconds such that if the hook switch stays at the
intermediate On-Hook state for less than this time limit,
the OBi device will not recognize the state transition as a
HOOK FLASH event, but consider the hook switch remains
at Off-Hook state throughout the transition (in other
words, the transition is discarded as a glitch if it happens
too quickly)