Cisco SPA120 & SPA122

This is similar to the older Linksys PAP2T ATA
Ensure software is updated, as this does help with the quality of the audio, we upgraded to 1.3.2-XU (014) Jul 2 2013
Blog post regarding this
Does seem to work will with the modems on Sky boxes to talk home on.

The screenshots here were taken from a SPA112 which was actually set up to give a Sky box a phone line to call home on.

In short, set it up without echo cancellation, a-law only, no fax detect, no jitter adjust (set to high), UK line impedance, zero gain, etc. PLEASE NOTE: If your line has a noticeable echo, turning on echo cancellation may assist. Do not use +44 as part of the username, otherwise the device will not register.

View of the 'quick settings'

Line Settings

SIP Settings

Change RTP Packet Size: to 0.020 (which is 20ms)

Regional Settings

Change FXS Output gain to -3 if modems have problems
Change the impedance setting to 370_630|310nF

Firewall & Security

You will also want to set up firewall rules, as per the VoIP Firewall page.
Also see the VoIP Security page for information about securing your VoIP service.