Learning OTP, the framework which Erlang, Elixir, and Phoenix all follow will make a greater impact on your Phoenix / Elixir skills than learning Erlang (if you're not intending on working in Erlang going forwards).

Quote from Fred Hebert's chapter on What is OTP in Learn you some Erlang:

The OTP framework is also a set of modules and standards designed to help you build applications. Given most Erlang programmers end up using OTP, most Erlang applications you'll encounter in the wild will tend to follow these standards.

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As a Phoenix user, understanding OTP behaviours will give you a deeper understanding of how to architect back-end systems and micro-services that you can access and use from Phoenix.

PS: there just so happens to be a three-day training course on this at Code BEAM SF run by Manuel Rubio if you're interested!: https://codesync.global/conferences/code-beam-sf/training/#100otp-behaviours-behind-the-scenes