It is working now, but only when both WANs have an IP address, even though only WAN2 is a backup (fail over)

The voice mail worked fine when correct port was opened and only one WAN was connected. Voice mail will not work when both WANs are connected COMCAST on WAN1 and local wireless internet on WAN 2....

Region : UnitedStates Model : TL-ER6120 Hardware Version : V1 Firmware Version : 1.0.0 Build 20130116 Rel.52243s ISP : COMCAST Greetings; I cannot get the send voice mail function of our NEC Apsire...