I realize this is 40+ year old technology. I have a question into my telco contact that's familiar with the AT&T 5ESS, which is somewhat different than ours. My customer's remote office has a few AT&T ISDN 8510T phones served from the local CO. The customer wants to forward their main number at times to another phone. The main number is not the primary line on an ISDN phone but is a SLO (Single Line Only) appearance on 2 phones. So, the typical procedure of 72#, (destination), wait for confirmation tone, press the CFV key does not apply.
She successfully forwarded the number by 174+(XXXX, the 4 digit extension). But, the code to cancel forwarding is unknown. In our system, an SLO is forwarded by 78#+ the 5 digit extension, 79# to cancel. The remote office gets a fast busy immediately when pressing 7, and 179+ 5 digits did not work. The CO serving the remote customer is a public CO, and I don't know if it's the same vintage as ours. My contact hasn't been able to research the SLO call forward cancel procedure yet, so I thought I'd give it a shot here.