5.10.1.6. Feature Activation when DTMF signaling is active 
Feature activations that use a keypad access code during local through-connect
states shall not work if the DTMF signaling is activated.
In the OpenScape 4000, when the DTMF signaling is activated on the
line, any access code will be considered as keypad dialed numbers and
will be converted into DTMF signaling.