Calling device in a digital communication network

For an inexpensive and intrinsically safe call system (1) a digital switching system (2) is provided, on the one hand with a Rufauslöseeinheit (7, 8, 9) comprising terminal (3, 3 ') and the other with an application server (6) data transfer standard connected is. The switching system (13) (2) further comprising a monitoring unit connected, which periodically sends test data to the application server (6) and, if not properly acknowledging the test data an error detector (15) enabled.