Try each in the MASTER socket and ring it with mobile.
Then try in each of your extension sockets. If no ring, then it may be that your extension sockets are only wired up for the line .. and NOT the ring line. Many digital phones do not need the ring line any more .. but it's worth a try.
The line is on connections 2 and 5 inside the MASTER socket (outlet connector to extensions) and into each extension (on term. 2 & 5)
The RING wire is on terminal 3. Try connecting a core to Master outlet 3 and again .. same core to extension terminal 3 .. and try that one.