Energy Centre calls

"I answered the call from this Mobile Number from an UNIDENTIFIED SOURCE, I said nothing at all, because I am 100% FED UP of RECEIVING CALLS from UNIDENTIFIED CALLERS ! This was probably another call from 'The Energy Centre' who I am NOT INTERESTED IN AT ALL ! N.B. Giff Gaff Voicemail DEFAULT SETTING is 15 Seconds of RINGING then GOES to Voicemail ! Giff Gaff SHOULD CHANGE THIS TO & HAVE 30 SECONDS of RINGING then GOES to Voicemail ! Giff Gaff NEED to CHANGE their DEFAULT Voicemail Settings as 15 SECONDS OF RINGING BEFORE VOICEMAIL IS MUCH TOO SHORT, as nobody ever leaves a Voicemail, & 15 SEVONDS is FAR TOO SHORT a Time Duration, for ME, to be ABLE TO ANSWER A CALL ! N.B. I DON'T GO AROUND with MY Smartphone CLAPPED TO MY EAR !"