I had a Philips VOIP 8551B and found it something of a triumph of style over substance. Yes, it looked nice and the colour display is good - but that was about it. The buttons were imprecise, the battery barely lasts a normal working day out of the charger and some of the ring tones don't stop if you answer on an extension phone (Philips confirmed this....).
Try updating the software (Settings / Advanced / Software Update / Check for Update), then log back in to your Skype account and wait?
I've tried all 3 of the available DECT/Skype 'computerless' phones:
RTX DUALphone 3088 - best landline performance, but the keypad buttons quickly lost their lettering, making it difficult to use. Rather a cheap and nasty feel to the handset.
Philips VOIP8551B - excellent Skype performance but poor landline performance due to a background buzz induced into my router by the USB connection. Handset looked nice, but keypad buttons were imprecise and battery life was poor.
GE1591 - monochrome display is very dated, but battery life is reasonable as a result. Handset functionality is more or less the same as the Philips handset, but keypad buttons have a far more positive action. Doesn't come with an answerphone, but probably the best of an indifferent bunch.
If only Panasonic or Siemens made a computerless DECT/Skype cordless phone!