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You have a couple of options that you should carefully consider.
Option 1) Send the phone to the manufacturer for repair. However, when you do you'll probably receive a notice that the handset should be replaced with a new one (because it is beyond repair). Manufacturers would rather sell you a new handset than repair an old one due to numerous reasons. And, the price will probably be much higher than purchasing the handset locally or from eBay.
Option 2) You're going to need to know the exact model number of your phone. There are MANY DECT 6.0 Digital Cordless Phone Models. And, you need the exact model number to replace the handset.
New, replacement handsets can be purchased on eBay for far less than you'd pay for one from the manufacturer. On most phones, a new handset is programmed to the base by setting the handset in the base (not the spare extension charger base - but the main base) for 20 seconds. But, read your user guide to make sure.
If you do elect to send your handset in for repair, (and it can be repaired) I've seen it take from one to two months to get one back. So, keep this in mind when making your decision. The address for service and repair can be found in your phone's owner's manual. Or, you can click on this link to visit the Vtech Support Web page: Vtech Product Support
Notice on their web page there isn't an actual link for repairs. As I wrote, they'd rather replace than repair it.
The actual cost for repairs won't be known until the handset is inspected by a qualified technician.
My suggestion is to save yourself a major headache and replace the handset either locally or from eBay.