2N Lift emergency phone 2N LiftNet - Manual, 1446 v1.8.3 - Manual (Page 58)

LN Programming 3.1
58
Today, voice transmission is prevailingly digital, using variable compression
algorithms. Therefore, the DTMF signal to be transmitted is often distorted. Moreover,
it may, in some cases, be transmitted through the so-called command channel, whose
delay may differ from that of the speech channel.
Caution
 Experience shows that, especially recently, it is practically impossible to
recover the DTMF signal in GSM networks
In such cases, try some other equipment (a digital PBX, e.g.) or the machine room
telephone set. If the machine room or PSTN programming attempts fail too, you have
probably entered an invalid password.