There are two possible reasons for the handsets not to ring when they are functioning properly. 1) You have the ring volume set to off. This would be done on each handset. With the phone in standby, press the volume up button until you reach the desired ring volume.
(There are 4 volume levels.)
2) You have toggled on the Do Not Disturb feature; this will apply to all of the handsets when started on one handset. Press and hold the # key (below 9) on a handset that is in standby. You should release the key when the phone stops showing the DND symbol. You can actually set the length of time in hours for the DND feature by pressing a number key (1-9) when the screen shows a prompt. If you don't press a number key within 5 seconds, the DND remains on until you press and hold the # key again.
If neither of the above solutions work, you need to check that the base is receiving calls properly. Check if a different phone (preferably a basic wired phone) rings properly when you get an incoming call. If another phone doesn't work, you'll need to check your wiring. If another phone works, contact Uniden if the XDECT8015 is under warranty.
Uniden has the manual for the XDECT8015 on the product page: http://www.uniden.com.au/australia/p_xdect8015_1_index.asp
. The direct link is http://www.uniden.com.au/RESOURCES_MAIN/pdfs/XDECT_8015_1_OM.pdf
I hope this helps.