You should be able to change the ringer settings in two ways when the handset is not in use. 1) Press and hold the Volume up/Down arrows on the handset. The handset should show the current ringer setting (High, Low, Ringer Off). Press Select. The current setting should flash and you will hear the current volume (if the ringer is on). Use the up/down arrows to select your desired setting; the phone should give a ring. Then press Select to confirm. Again the phone should ring.
2) Press Prog. Use the up/down arrows to select Ringer Volume. Then press Select and use the up/down arrows to chose a new volume. Press Select to confirm.
If your phone still does not ring, there are items to check.
Try a new telephone line cord and make sure that the base is plugged into a working outlet. Move closer to the base.
Make sure that you are away from other electronics that could interfere. Check that you do not have too many devices plugged into your phone line. Each one should have an REN (ring equivalence number). The total REN for all devices should be 5 or less; disconnect the other extensions/devices and test again.
Make sure that the phone is providing a dial tone and a test phone will ring when dialed. (I like to use an old corded phone for testing.) If a test phone won't ring, you have a wiring problem. See additional trouble-shooting information below.
Remove the battery in the handset and then reconnect it. Wait for the handset to synch with the base and try again.
If you have a DSL filter, try replacing it.
The manual for the CS5121 is available from V-Tech here: http://www.vtechphones.com/vtechphones/index.cfm/product-detail/?event=ehCatalog.productDetail&ProductID=55#documents
Phone line testing:
Using a test phone with a new line cord, check for dial tone and ring at the first jack. If your test phone rings, then the problem is the handset. Move to another jack on the line and test again. If you get a ring here, the first jack or the wiring to the jack has a problem. If there is no ring, find where your service comes in. Open the box and check the type of connection. If you have a relatively new phone service, you'll find an RJ-11 jack. Plug in your test phone and check for dial tone and ring. If it doesn't work or there is no RJ-11 jack, contact your service provider. They can test the line and will repair any outside problems for free. If the problem is inside, you will need to look at all the exposed wiring for damage. If only the one jack is bad, open it up and check that the wires are properly connected. There are two types of jacks: one secures the conductors with screws, the other is a keystone. The keystone jacks require a punch down tool to press in the conductors. This punchdown tool is available from many home improvement stores in the wiring section. This site has a picture of how the conductors should be connected: http://www.lanshack.com/wire_phone_jack.aspx
If the handset is bad, you can try a replacement handset, if available. If the phone is under warranty, contact V-tech. Otherwise decide if it is worth servicing, these phones were not meant to be user serviced.
I hope this helps.