you probably already known how to,
but I'll say how I would check a digi.
test butt on monitior, when you toggle an output on com2400 you'll hear it dial the no. then receiver answer, the data sent & the acknoledgement which tell the digi it's successful & to stop.
Adsl, funny dial tones ect might stop the receiver being heard, the digi will normally try multiple times.
The digi also needs to be enabled in the menu so it works
It's hard to work out digi issues without a telephone handset / butt another basic test is dial the receiver off the line & see if the receiver answers