Australian law might differ significantly to what I'm familiar with but my understanding is your two points here might not be true for someone who simply refuses to leave when asked:
I don't think a regular sales person has the authority to declare a customer as a trespasser. Wouldn't that have to come from the land owner - ie someone senior within the organisation? (assuming Telstra own the land - which isn't usually the case with shops anyway). There typically requires some formal warning too (I don't think a verbal "I'll call the police" counts, but I might be wrong). Plus if Telstra do own the land and then call someone senior then you've still won because the whole point of the sit in was to speak to a manager.
Also trespassing isn't automatically a crime. Not all offences are criminal offences thus you can be breaking the law without committing a crime. Trespassing is one of those laws that can be a criminal offence but it's not automatically one. I very much doubt someone politely refusing to leave is automatically a criminal offence.
> That is not a protest, it is just exhibiting a shocking lack of self-awareness and -- trespassing.
The two aren't mutually exclusive.
The issue is you are literally trespassing at that point. It is illegal, you are committing a crime.
You can't take the law into your own hands. If you are "legally owed" a refund, you can take that to court, prove it, and receive judgment for your refund.
What you cannot do is judge for yourself that you are legally owed a refund and therefore entitled to stay in a retail store regardless of whether or not you are wanted there. That is not a protest, it is just exhibiting a shocking lack of self-awareness and -- trespassing.