Found user with butt set on an MGCP controlled VIC3-FXS/DID port can break DT and dial a number without issue. When a Sonitrol alarm panel is connected to the same port and dials the same number, either DT cannot be broken or number cannot be completed as dialed.
Fix was to reduce the input gain on the FXS port via CLI. This cannot be adjusted via CUCM like an FXO port, but is still an available option. Show voice port X/X/X indicates the default gain is 0db.
Working configuration is:
input gain -6
description << Sonitrol Alarm >>