Sounds like the base is faulty and you need a new one.
You can buy a replacement BT Freelance XD5500 base from sellers on eBay for £3.99 for the base and £3.99 postage & packing..
http://stores.shop.ebay.co.uk/AK-Products-UK_BT-Replacement-Parts_390115306903_W0QQ_fsubZ11329509QQ_sidZ213380995QQ_trksidZp4634Q2ec0Q2em322
Upon purchase you need to reset the base by following the instructions below:-
BT Freelance XD 5500
Resetting the base & handset/s on a BT Freelance XD5500:
Hold the Find button on the base unit until you get a double beep then 
tap the Find button 7 times. 
On the handset: 
remove a battery 
hold the menu key and refit the battery 
screen says SW VERSION 
press up arrow key until screen shows REGISTER CLEAR 
press menu 
press menu again and wait for OK to appear 
remove a battery and refit 
Registering the XD5500 handset.
Then register the handset to the base as follows: 
Hold Find button on base unit until you get two beeps 
On handset: 
Press menu 
Scroll to register 
Press menu 
Scroll to Register Base 
Press menu 
Press 1 
Wait 
When it says PIN ? Enter 0 0 0 0 (four zeros) and press menu. 
Note:
If you have more than one handset then repeat registering the handset to the base for each subsequent handset/phone.
If this procedure fails to solve the issue, then you have a unit with a  fault and if it is less than a year since purchase the retailer should  change it for you.
                
 Kevan
                
 November 2009