I called back number (03) 7000 5428 reached voice mail but did not say for what company.

This was a missed number on my phone.
After blocking my number from being sent I rang the number back and muted my phone.
Call went through to voice mail which asked me to "leave a message for extension 5428".
At this point I terminated the call.
Based on previous experience with auto diallers used by telemarketers, I believe this to be a telemarketer or scammer who is attempting to get people to leave call back details.
If you leave any details (and, remember, if you haven't blocked your number from being sent then it will be recorded on their system) this then tells them that they have dialled a 'live' contact, and then they will start to harass you.