If you are considering that kind of money, I would recommend the AVM (
http://www.avm.de) Fritz!Box Fon Wlan for about £130 including VAT/Delivery....
Basically it is an ADSL modem, router, wireless access point, with some very interesting telephone (almost PABX like) features on it....
Connect your landline from the ADSL splitter to the box, then connect your DECT cordless phone basestation to the phone out and voila you have both landline and Voip calls on your normal phone....You can configure it for automatic pre-dialer configuration and all sorts of rules...Mine for example switches automatically to VoIP out via Voipstunt when a number begins with 00 (internal calls).....Also I have my fax connected via a DECT wireless phone socket and incomming fax calls to my second Sipgate (free regional number) go automatically and only to my fax...Etc.....
There are other high-end ATA's that can do this, like the Linksys 3000 I think, but that is (to me) yet again another box, another powersupply and doesn't have the automatic traffic shaping (QOS, necessary to both download and talk) thus making it more complex to configure from a normal router or switch....
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