I will endorse the Gigaset range too. Are you looking at Business or Residential or don't mind?
There is a NEW product in several variants which includes the first handset: S850A GO - C430A GO - SL450A GO and a few more with the GO. It is an upgraded version of the N300AIP. The original N300 and N510 are the exact same product with different options enabled in the firmware however it is apparently not possible to convert one to the other.
A list of them is available here:
http://www.gigaset.com/hq_en/telephones-dect/cordles...
Their compatibility list is at:
http://www.gigaset.com/hq_en/cms/home/support/suppor...
I have just purchased a new S850A GO to work with both VOIP and PSTN - no operational issues with it. One minor one setting up BT BBV services but that is resolved. I can use the handset with a wired or bluetooth headset too.
And if you want a well priced source - PM me and I will send the details through (prefer not to advertise a company I deal with here).
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