WARNING message: Channel 'mISDN/1-u0' sent into invalid extension

My log file (/var/log/asterisk/full) contains lots of these entries:

WARNING[19236] pbx.c: Channel 'mISDN/1-u0' sent into invalid extension '123' in context 'from-pstn', but no invalid handler

where '123' is one of our phone numbers.

its interesting to note, that we don't use "from-pstn" anywhere, since we only have ISDN lines connected to Beronet cards.

Any help would be appreciated.