Connected Outside Line Presentation

Connected Outside Line Presentation - COLP

Does anyone know exactly what this is?

I believe it is where the Called Party Number is returned to the Calling Party on answer so that if the call is diverted the actual answering number is displayed rather than the dialled number.

COLP is used in ETSI ISDN documents but the O is not Outside but COnnected Line Presentation

A BT ISDN30 Doc says

Connected Line Identity Presentation
(COLP)
Allows you to confirm that the number
you have dialled is the number you are
connected to and that you have not been
forwarded to another location.
Activation/deactivation of this facility is
not available on a per call basis.

Thanks ... that explains why my searches for "Connected Outside Line Presentation" were failing - the handset manufacturer used that name.

However, it seems to confirm my understanding of it.

it does indeed confirm. I saw it on the Gigaset site with "outside" included which may make more sense with a PABX or DECT when there is an outside.