SIP Problems with ZyWall 310

We are using cloud-based VoIP phones, from OnSIP. The phones are on a separate port with DCHP running from the zywall itself,  but they randomly lose connection and they are not reachable from within or from outside of the network. We are using Firmware Version: V4.31(AAAB.0) / 2018-03-27 17:03:23 ,ALG is disabled as can be seen in the screenshot

 It was suggested by OnSIP support we put 'access rules' for a few subnets and so we did. Added the subnets in Configuration -> Object -> Addresses/Geo IP
then created security policies to allow the incoming traffic from the said subnets from WAN to the Tel_Corp(zone -LAN)

and yet we are still having sporadic connections and I can see the packages being dropped  in the log 


It would be greatly appreciated if this problem gets sorted out because this is affecting our business and the support we received so far from ZyXel far from good.
Thanks