only Transfer button or all of CAD buttons?
Can you please set the CAD level to DEBUG and share logs?
A shot in the dark...
Did you upgrade call manager recently or phone firmware? The phone firmware that was the latest until a week ago has a bug that effects transferring calls when using a headset.
If it turns out this only effects those with a headset, youre likely on firmware 9.3(1)SR4 look to upgrade your 794x/6x phones to the 9.4(2) firmware or downgrade it.