Add a config check to hold phone extention

Add a config check to hold phone extention

Rather than returning the hold notification phone number
on every checkin response, use the phone_ext_to_msg10
institution setting to enable this.

Signed-off-by: Jason Boyer <jboyer@equinoxinitiative.org>
Signed-off-by: Martha Driscoll <driscoll@noblenet.org>