We use REST for both internal and external calls to services (a mistake, in my opinion) and while Erlang actually has frameworks for this, none of them are good and cowboy_rest requires lots of boilerplate. Doing it with Phoenix would cut out lots of entire modules and replace them with functions without paying any real cost.
I like Erlang, but there's no upside to using it instead of something else that runs on the BEAM, apart from making a basic library that you want to use both from Erlang and Elixir (I don't know what the situation for compiling Elixir with rebar3 is).
Effectively, Erlang is to Elixir what Java is to Kotlin, except there are even less performance differences and no compatibility issues. It makes perfect sense to learn Erlang syntax and learning the semantics of it will mean you've learned BEAM semantics, but you might as well do that in Elixir and actually have a better road to it.
Edit: I'd like to add that I find a general lack of tools for dealing with boilerplate in Erlang. It also has a generally outdated functional programming skew in that it doesn't at all facilitate pipelining and this makes it less attractive to make nice abstractions like Plug(0), for example. This is exacerbated by the fact that collections aren't abstracted over, so whenever you end up dealing with them there's lots of sighing and docs checking to see what the argument order was the week they designed that particular module and that particular function in that particular collection module.
0 - https://github.com/elixir-plug/plug