when talking, cannot pick up other calls from other phone sets

[Poinciana]

Hello everybody
I am running 5 Obi 1062 phone sets. Let me name A to E

When I made a call from phone A (or other), and other calls come in, all other phone sets would not ring. However, if I used phone B to pick up the call, when third call came in, only phone B would ring, and other phone sets (C,D,E)would not ring.

When I answered a call from A (or other), and other calls came in, only phone A would show up a phone call, and other phone would not ring.

It didn't happen before GV upgraded to SIP.
All phone sets have been upgraded to latest firmware. 

Please help.

Thanks a lot

[SteveInWA]

Google Voice is not a commercial telephone carrier.  It is not intended for your multiple-call use case.  Subscribe to a business-class, paid SIP VoIP Internet Telephone Service Provider.  There are many options available, from DIY for tech-savvy users, e.g. Callcentric, voip.ms, or turnkey, full-service offerings, e.g. Dialpad or Ringcentral or Phonepower.