It sounds like you're describing interleaving - if you have a longer/lower quality phone line applying interleaving can help stabilise your line and overcome errors with the effect of slightly increasing latency.
There are 3 settings that can be applied: Auto (let BTs line management system decide to apply interleaving or not), On or Off. As a side note - on 21CN exchanges the amount of interleaving applied to the up and downstream can be made independently of each other.
If you call Zen tech support we can apply any of the three main options you prefer, although changes are made at your own risk (if you ask for interleaving switched off on a 'marginal' quality line against our advice and your connection goes haywire, don't blame us!) although we can revert any changes that are made if requested.
For the vast majority of lines the default 'Auto' setting works fine - if you have a good enough quality line it will set interleaving to 'off' anyway.
Zen Technical Support