I recently ported my UK landline to Andrews & Arnold's VOIP service. I was running a Grandstream HT801 connected to my ASUS BT8 router for my previous Vodafone VOIP service but saw that there was an upgrade available for the HT801, imaginatively called the HT801v2. It has a much faster processor and a completely new GUI. I couldn't find any online guides to setting up the v2 but after a good deal of trial and error I have it working. I thought it might save others a lot of time and hassle if I posted screenshots from my configuration. You also need to disable SIP ALG, SIP helper features and UPNP in your router settings.