I have had pretty good luck with something like this:
ephone-dn 12
number *111
label Intercom Ken
intercom 111 barge-in no-mute
Then map the dn to a button or overlay it with another.
ephone 7
mac-address 0000.0000.0000
username "ken" password 12345
type 524G
button 1o2,12
Hope this helps.