Why do some icons from fontawesome in mermaid work and some do not?

In a revealjs quarto presentation file using Rstudio, when I do:

```{mermaid}
flowchart LR
    D(fa:fa-phone)-->C[fa:fa-house]
```

and knit the file, I get this in the output:

Why does the first icon work, and the second icon not?