Presently on wireless broadband with B618 and Panasonic KX-TGC220.
Mostly 50-100Mbps with occasional 20ish, and 130Mbps.

Due to some long past experiences with persistent nuisance calls we now screen everything with answerphone.
This works well, everyone in the house is not jumping when phone rings.

If we hear a friendly voice answering we can pick up.
Spam calls hang up upon answerphone picking up.

Recently calls from various 0955878xx numbers behave differently.
Answerphone does not answer these.
Apart from first normal double ring, ring tone is a different single ring thereafter. 
This keeps going until they give up. So beyond number of rings when answerphone usually activates.

Is this a known thing telemarketers have to disable answerphone, a code sent along with the caller id ? 
Elsewhere similar numbers are reported as Mondial telemarketer.

No big deal just curious if anyone knows more.