Check with the developer, but there isn’t in effect a master socket , if you have several phone extension sockets in various rooms , they could be ‘daisy chained’ together, so extension 1 is connected to extension 2 , extension 2 connects to extension 3 ( you wouldn’t necessarily know which is the first extension ) , or they could be individually wired back to a common point , in some respects it doesn’t matter , with ADSL/VDSL incorrectly or poorly wired extensions connected to the copper pair into the property, can impact broadband performance, with FTTP it’s irrelevant, you don’t have a copper pair, the master socket was the demarcation point , than is now the ONT.
If you are taking DV with your FTTP broadband, you may be able to incorporate these extension sockets by connecting the router phone port into one , and using the others , but would only be of use if you intended to use wired ‘landline’ phones , or wanted to use cordless , but not use the SH2 as the DECT base station and have your own base station located away from the router
