Depending upon what is required to trigger the Alarm , if its a simple relay activation then
I would look at something external like an Algo 8180 sip device , program it as a generic sip extension in mitel , program it via its web interface , relay can be activated when rung .
As mentioned though without more system details its hard to offer other options.
There are lots of other options like door stations that have relay contacts as well
If I never did anything I'd never done before , I'd never do anything.....