I was initially against this idea because I felt getting as simple a device as possible would be the best approach and I didn't want another bulky router when it wasn't needed. However reading through the reviews of the Grandstream it does seem a bit and miss to get working which is my experience with similar devices at work so I am starting to lean towards the router ideaI would go for a Fritzbox 7530, it has two analogue phone ports plus built-in DECT basestation for up to six handsets. Also, you get dual band wifi and a lot of other features. You get one thrown in with Zen Internet or second hand on eBay from £40 onwards.
I have been using a 7270 for years and upgraded to a 7590 last year, still on ADSL2+, desperately waiting for FTTP to arrive.
You could also look at the ZTE ZXHN H298A, two phone ports and support for 5 DECT handsets. They sell on eBay for around £10, might be worth a gamble.