Peplink Balance one and Polycom realpresence 500

If i connect the lan cable directly to modem,
Outgoing video conference - Working Fine.
Incoming video conference - Working Fine.

If i connect the lan cable to Peplink, then from peplink to modem
Outgoing video conference - No ringing. Cannot call
Incoming video conference - Working Fine.

Peplink Balance one Configuration :

Inbound Policies
-Nil

Outbound Policies
-Nil

Nat Mappings :
Lan Clients : 192.168.4.5
Inbound Mapping : (LAN 2 as WAN 4):58.26.xx.xxx
Outbound Mapping : (LAN 2 as WAN 4):58.26.xx.xxx

Service Pass through
-Enabled H323

Please help me.