If you will get , as a consequence of migration to a PDPL (pre digital phone line) some ‘star services’ (as they were once called) ,for free ( ones you would have paid for on the current PSTN ) like call divert etc , why would you assume you won’t keep a star service you already have and use , the ability to make more than 1 call simultaneously ?, seems an odd assumption, surely the assumption would be that it remains available, after all its potentially revenue earning .
I don’t know for sure , I don’t have a PDPL , but it’s most likely you will continue to have multi call or whatever it’s called , in the unlikely event you don’t keep this facility , removing something that was ‘free’ and giving you other previously paid for things should not be cause for complaint anyway …the obvious thing to do is when you get moved to PDPL , give it a try .
