2 concurrent call over 1 ISDN line


#1

Our asterisk is attached to our old PBX via 1 ISDN line.
I can dial out and dial in on the same time over that channel. (ISDN1)

When i try to dial out with 2 telephone at the same time i recieve following error:

didn’t find capi device for interface 'ISDN1’
cause 44 - requested channel not available

What am i doing wrong. Can somebody help?