I thought we'd mutually figured all this out. I'd thought this is what we'd been telling people on OzBargain. And Google didn't find me any references to the contrary.
A SIM from any phone company that uses the Vodafone network will work in a Vodafone locked phone.
A SIM from any phone company that uses the Optus network will work in an Optus locked phone.
But I've just found that an Aldi SIM will not work in a Telstra locked phone. Maybe its something to do with Telstra having a wholesale network and a retail network.
So I've wasted the cost of what seemed like a bargain new second phone from Woolworths for very occasional calls that I'd intended putting a PAYG SIM in. All $9. And the $5 for the Aldi SIM.
100% this. Boost = full Telstra, Aldi = Wholesale Telstra