Ther most common cause is noise on your line. You can do a quiet line test, 17070 option 2, best with corded phone. The line should be silent between announcements, you may get a dull hum with a cordless.
log into your modem using 192.168.1.254
then connection and see what uptime is for vdsl if it shows its dropping every few hours have a listen to the phone any noise ?
then check in the test socket of the nte see if same or if you are running on an extension.
then look at the snr on modem see if its flapping if so suspect a HR on the line