It appears the C40 is registered to CUCM, and looking at the logs, the following two lines caught my eye.
2016-06-09T09:03:09.066-05:00 ppc appl[2775]: 609422.37 SipReg I: New active CUCM '10.99.3.11', registration URI '81717236@10.36.40.146'
2016-06-09T09:03:27.072-05:00 ppc appl[2775]: 609440.38 SipReg I: New active CUCM '10.36.40.146', registration URI ''
It appears whenever it is uses CUCM with IP 10.99.3.11, it obtains a URI, however whenever it uses 10.36.40.146, it doesn't. I would check the provisioning settings on the C40 whenever this happens, making sure that they are correct for the CUCM it's supposed to be registered to.