TESCO/Siemans DP 450

Hi,
Can anybody out there offer me some advise please? I bought a DP450 (and an additional DP45) from TESCO last Novemebr and have been trying in vain evr since to get it working. I have spoken to the helpdesk(in Australia) loads of times but their solution is for me to email a copy of my reciept to get my money back. This is because they say the DP450 is faulty but I wish to perservure with the service as i live in Cyprus and having my parents call me so cheaply is a definate bonus for them as they are'nt the richest of people. The problem is that I can use the phone to dial out but all incoming calls by pass the phone and go straight to my computer. I have previously had a USB handset and an IPA 1000 on the TESCO service. With me able to make outgoing calls and all incoming calls goint straight to the computer (and hence to voicemail). I suspect that the DP450 is not U/S but just a configuration problem-any ideas please??
Mick