I called back number (07) 3505 5911 reached voice mail but did not say for what company.

????? me too

they didn't leave a message. called number back on private - no company name. "leave your name & number and we will be delighted to give you a call back".
Don't call them or leave your details

telemarketing - want me to change energy provider, wouldt state where they got my number from.

Dont call back just ignore. not important,

Said it was Oz Green who are an Energy company. Very nice man, not your normal telemarketer!

I don't know who this is, no company name when the phone is answered., only a message bank. If anyone has a clue who they are please let us know thank you.