Didn't leave a voicemail, so assuming it's spam.

Caller: (02) 9129 6708

The phone number (02) 9129 6708 has been reported as a suspicious telemarketer promoting solar panel sales. Learn how to handle calls from this number and protect yourself from potential spam.

Caller: a solar system battery offer

called back to find out who this would be. told me that it is a solar system company to offer special deal for solar battery.

Tagged as potentially fraudulent on mobile, left no message, assumed spam (or worse).

Called and rung out after a few seconds, didn't call back, no follow up text, etc. I suspect spam call. Don't answer unknown numbers and don't call them back.