We have successfully tested the pbx using 4 ISDN channels at the once in our office.

However when we take the pbx down to a clients who recently had ISDN installed (4 channels, 2 BT Boxes same as ours) then we can only make use of 2 channels at a time.

In online status section in the pbx it says that the 2 Trunks (ISDN1-D and ISDN2-D) are UP.

Ive also made sure that in the digit map setting that ISDN1 is the primary route and ISDN2 is the backup route.

Does snyone have any suggestions as to why we can still only use 2 channels at a time?

is there something that needs done on BT's end?

Any input would be much appreciated

Does it work outside digitmap?

First two calls dial 903 and then dial the destination numbe after hearing the isdn dial tone.

For call 3/4 dial 904

Also, check if the lines are configured to point-to-point or point-to-multipoint mode.