Trixbox "No application ADSIProg" can't program Aastra 390

I have set-up an extension in extensions_custom.conf like this:

exten => 998,1,ADSIProg(asterisk.adsi)
exten => 998,2,Hangup

However when I call it just hangs-up. The Asterisk log shows this:

[May 23 23:45:35] WARNING[20200] pbx.c: No application 'ADSIProg' for extension (from-internal, 998, 1)
[May 23 23:45:35] VERBOSE[20200] logger.c: == Spawn extension (from-internal, 998, 1) exited non-zero on 'Zap/1-1'

Any ideas to why it can't find the ADSIProg application? Also, when I call Voice Mail it doesn't try and load in the Comedian Mail script either (most likely the same problem).