Add basic test coverage over SMS Phone Number submodule

Problem/Motivation

It would be nice to get a stable release out for this module, but I am unsure if our SMS Phone Number submodule is functional/stable. It was functional at one point and there is some evidence in our issue queue of community members currently using of it, so I suspect it works.

Let's confirm our SMS Phone Number submodule works with some functional tests. If there are broken or buggy parts, these tests will help illuminate them so we can fix them.

Steps to reproduce

N/A.

Proposed resolution

Add basic test coverage over our SMS Phone Number submodule showing that its basic functions and integrations work as expected.

Likely the modules it integrates with will include test coverage of their own that we could refer to or even lean on when creating our tests in here.

Remaining tasks

MR + Review.

User interface changes

None.

API changes

None.

Data model changes

None.