hello Tarun,
these messages usually are CDP derived.
Another thought is that speed and duplex are physical properties that can be configured only on the physical interface gi0/1.
So a complain for duplex mismatch is a true issue only if it is seen in
sh int gi0/1.
Then the IP phone should be the first device out the port.
In this case you should change the cable or verify cabling it can be a cable issue
or access via web the ip phone page and check for collisions errors or other signs that is putting the link in half duplex.
Hope to help
Giuseppe