Often BC6000 issues can be caused by weak batteries - are they all relatively new?  Turn all handsets off first and then try each in turn.

Also, have you tried another (non B&O) phone in the socket?  I wouldn't rule out a fault on the line.  So few people use landlines these days that landline faults can go a while before being reported.

The other thing that you could try is a complete system reset - each handset and the base.  Be aware that you will lose stored numbers if you reset the base.

Thanks Guy

Batteries are relatively new and show as full in the handsets' settings.

I have followed your advice and tried another phone on the same line. The other phone dials out fine.

Next I tried the B&O base on the second land line and it has the same issue as on the first line.

So the issue seems to be with the B&O base. I am not very hopeful a complete system reset will solve the issue. Looks more like a fundamental fault to me. I will try to get another base second hand.