Trying to flog courses in something. Not interested

Constant calls form 1183, 1185 and 1187, twice in a row over 3 - 4 days, no message left, blocked.

Receiving 2-6 calls a day from (02) 8527 1184 / 1185 & 1187 over the last week. No message left.

This number rang me four times yesterday and twice today (so far) with a slightly different number (ending in 3 instead of 7) I've developed a policy answering numbers I don't know but staying silent until they hang up after 45 seconds (presumably costing them time and money). It was a real person who knew my formal name but not my familiar name.