DPA Section 11.
An individual is entitled at any time by notice in writing to a data controller to require the data controller at the end of such period as is reasonable in the circumstances to cease, or not to begin, processing for the purposes of direct marketing personal data in respect of which he is the data subject....
with the all encompassing caveat that ...
“direct marketing” means the communication (by whatever means) of any advertising or marketing material which is directed to particular individuals.
Or in other words, if you've sent BT a DPA section 11 notice and they do this to you they are breaking the law. Yet again.
If you want to send BT (or indeed any other UK telco) an ICO recommended DPA section 11 notice, use this wizard to generate a letter. Print, post, send...
https://www.dephormation.org.uk/dpa_notices/
If you still receive unwanted marketing voicemail spam, complain to the ICO and (eventually) it will stop.