This is from the BT web site so obviously not the foremost knowledge on ISDN but, you know
There are two ways of implementing call forwarding on ISDN2e:
Admin Controlled Call Forwarding
Admin Controlled Call Forwarding (ACCF) is an exchange-controlled call forwarding option. This means that it does not require special equipment. To set up this service call Business Sales on 0800 800 152. The service is active within 24 hours.
Customer Controlled Call Forwarding
Customer Controlled Call Forwarding (CCCF) diverts calls to all numbers to a single number of your choosing. This call-forwarding option is applied by you by programming your equipment to divert the calls. This means it is exchange-independent. Contact your equipment manufacturers if you are unsure whether your equipment supports the service. To enable call forwarding on the line, ring Business Sales on 0800 800 152.
Please note: The forwarded leg of the call is chargeable. The charges appear on your bill along with the subscription for the forwarding service.
The following options are available for both Admin- and Customer-Controlled forwarding:
Call Forwarding Unconditional (CFU)
All incoming calls are immediately forwarded to the nominated destination number.
Call Forwarding on No Reply (CFNR)
All calls are forwarded to the nominated destination number if the call is not answered within approximately 20 seconds.
Call Forwarding on Busy (CFB)
All calls are forwarded to the nominated destination number if the line is engaged.
Please note: If you have an ISDN2e line with MSNs present, the call forwarding is configurable on a per MSN basis. This is not possible with admin-provided call forwarding, and also not possible if you are using DDI numbering.
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What is the difference between Customer Controlled Call Forwarding (CCCF) and Admin Controlled Call Forwarding (ACCF)?
The two services do the same thing, by diverting all your numbers to a single number of your choosing. However, the way in which the divert is applied differs greatly:
ACCF requires a call to Business Sales on 0800 800 152 to specify the number to which you want all your calls diverted. Calls are diverted from the exchange and do not require you to have compatible equipment installed at your premises.
CCCF diverts all calls to a single number of your choosing. This again diverts calls from the exchange, but rather than having BT turn it on and off, you can control it from your equipment. This service has to be enabled on the line in order for your equipment to communicate with the exchange and divert the call. Prior to subscription to the service, check with your equipment manufacturers that the service is supported. To order CCCF, call Business Sales on 0800 800 152.