DHCP on WAN renews every 2 hours

Hoping someone may be able to point me in the right direction.

I have a Fortigate 90D connected to an ISP that is providing a static IP by using a reservation within their DHCP services.  The fortigate WAN connection is simply configured to obtain an IP via DHCP.  Easy config.

But, I've noticed in the logs that the fortigate is renewing the IP on that interface, consistently every 2 hours.

I've been in touch with the ISP and they say that their system have the lease set for every 4 hours.  (why every 4 hours when we have a reservation for a fixed IP is beyond me).  

So my question is, is the fortigate doing something, every 2 hours, and it is consistently on the exact hour, every 2 hours that would force a lease renewal?.

I'm monitoring the WAN connection using Zenoss and I don't see that link actually being affected by this, however, it has been noted that the hosted VoIP telephone system "drops" calls consistently throughout the day.  Even though I don't see the link dropping, maybe it is affecting traffic in some way.

Any thoughts would be greatly appreciated.

Oops, by the way, I upgraded to the latest OS last night.  5.2.4 and still seeing the issue.