It may or may not have anything to do with the way you connect via an extension socket. You would have to eliminate it from the set up before you could be sure.
You need to connect your Homehub directly into the test socket on the master socket to see if things improve. If not it could be a problem with your line.
Before doing anything you need to give information on whether you have ADSL broadband or VDSL broadband.
Can you run this checker and post back a screen shot of the results. Make sure you delete your phone number before posting.
If you are on VDSL broadband if you have a Homehub 5 can you post the stats from 1-12 by logging onto the Homehub management pages then troubleshooting > helpdesk. http://bthomehub.home/
If you have the Smart Hub, (Homehub 6) can you go to hub manager then advanced then technical log and post the stats. http://bthomehub.home/
Is there any noise on your phone line? Dial 17070 and select option 2, there should be no noise between the announcements.