In your WAN setttings (ZyXEL Router), is your connection set to Nailed or is it Disconnect On Idle (or similar)?
Also it would be of big help if you could post your line stats. Downstream and Upstream SN/Noise Margin and Line Attenuation. You'll find these in the Diagnostics section of your ZyXEL Router.
Also, check out:
Answer
Make sure your equipment is in line with what this support page from BT outlines.