Call Home in NX-OS and IOS use code outside the call home code base to create and send call home messages. One part of the code it uses is the SNMP MIB to store information required for call home messages.
The design goal of Call Home is to identify the person to contact, how to contact them, and where to send the RMA when Call Home sends in a message. It needs this information when building a call home message. It uses the following to do that:
1. sysContact (Required in standard MIB)
2. sysContactEmail (Required in Call-Home MIB)
3. sysContactPhoneNumber (Optional in Call-Home MIB)
But when Smart Call Home opens a TAC case due to a call home message being received, it actually uses the Cisco.com ID profile associated to the device to fill in these fields in the TAC case. But if you were going to automatically troubleshoot registration issues without a TAC case though, you would have to rely on the information in the Call Home message.
I'm not sure if the IOS call home code just doesn't verify that the sysContact exists before it starts functioning since it is set with a non call-home command [snmp-server contact], or IOS made sysContact optional. With Anonymous Smart Call Home, I can see where sysContact might be optional but so would sysContactEmail.
NX-OS requires and verifies that sysContact exists for Call Home to function.
