Regarding the ENP of VEGA-50G

As the doc said, the ENP should works with FXSs and SIP phones, I follow the link and do some test with ENP:
I found the ENP could not work properly. the results are:

  1. When PBX down, the sip accounts can re-register to VEGA proxy and SIP phones can call each other.
  2. When PBX down, the sip phones can make out going call through VEGA-FXO, the mobile rings, but we can not hear each other.
  3. Some SIP brands works(fanvil), some maybe not(GS).
  4. When the FreePBX is up, the SIP phone can work and re-regiester as normal, all phones work well with outgoing calls and incoming calls.

any idea?