That sort of thing happens —- try and be really clear what you expect back if you do an assignment. Put it on an email. Otherwise I’d write to them and tell them that you have an issue. Chase a bit, presumably you don’t know the reason for the ghosting. Having said that they could be ghosting you because they are assholes. There are unfortunately a lot of inconsiderate assholes in tech.
There’s also a lot of people that think the world owes them something. Why do they owe you a response? OP agreed to do an assignment did but did they get the company to agree to call them back on a set date? A good job is like a hot girl they don’t have time to text back all the rejects
Asking someone to do so much work for you and dismiss them is a dick move in any culture.