Sip Phone behind 2 WAN (load balancing)


We have Sip-Phones behind a 2 WAN load balancing area.
If I go to a website, who show me my active IP address, there will be sometime x.x.x.15 and sometime x.x.x.16.

A Phone register on an external Asterisk-Server.

‘sip show peers’ show
User = 200 Host = x.x.x.15 Status = OK
somewath later
User = 200 Host = x.x.x.16 Status = unreachable

If I reboot the phone, status will be OK again.

How can I handle this behind a load balancing?