All the existence of a ‘master socket’ indicates is that the property has had working service at some point in the past , it may have been recently in use , or could have been ages ago , there shouldn’t be any need to fit another master socket, or do any ‘internal’ work that may affect your neighbours, the socket and wiring to ‘outside’ should be OK to re use.
A ‘new line’ doesn’t necessarily mean that everything has to be provided from scratch, although there will be some ‘new lines’ that are the first time the property will have been connected , the majority will reuse what already exists.
If the previous connection on that socket still exists ( just not in use ) then it’s likely that your service can be provided without a visit to the property at all , any work for the ‘engineer’ would be done outside , you may just get a text stating’your line should now be working’ but because there is a possibility that engineer access to the property could be required, you are asked to be available so access can be given.
If you have received your router etc, and haven’t already done so , you could connect it now , it’s possible that your broadband service will ( from your point of view ) simply start to work , because some external work has already been done, using the ‘routing’ information from the previous user at your address , and Openreach used that info to connect your BT service,
