Phones are being discovered in only one of the two servers in the cluster. CCM 5.0. SNMP credentials are ok in both servers.
CUOM discovers the phones by doing a snmpwalk on the CUCM. Please check the following.
Make sure the capability listed for the CUCM says "CiscoCallManager".
CUOM does a snmpwalk on the CUCM for the object ccmphonetable (.184.108.40.206.220.127.116.11.18.104.22.168). Check if that response is giving valid information on the phones. If the CUCM is not giving valid information on the phones then this is a CUCM bug and you should open a TAC case so we can look into it.
Also check if the object "ccmPhoneProductTypeIndex" is returning 0. If it is returning 0 then try restarting the snmp service on the CUCM. If it is still returning 0 after that then it is a bug on the CUCM as 0 means unknown phone model type which causes CUOM to not discover the phones. If this is the situation please open a TAC case so we can look into this.