Impossible to say for certain without trying a replacement, but difficult to imagine it's anything else. A quick look at that auction site shows the same model for sale around £15-20. So if you're otherwise happy with the phone, maybe pick up another to try. If that works you could always re-register your existing phone to the "new" base & just use it as a second handset.
Or look to upgrade to a later version from somewhere with a no quibble return policy like Amazon or Argos. If there's still an issue you can then just return.
Although cheapest of all would be to just turn it off & use the free BT Voicemail.
