Specify telephony service in specs

[monfresh]

Why: To make sure the correct class is called each time. Otherwise,
a class that was set by a previous spec will still be in effect for the
next test, but might be the wrong class for that test.

[monfresh]

This was failing the Circle CI run for my other PR: https://circleci.com/gh/18F/identity-idp/193?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[zachmargolis]

This kind of global mutable state is usually a smell, should we change how this is set (ex make it a constructor param) or add some before-each stuff to the suite to reset to a known value?

[zachmargolis]

LGTM thanks for catching!

[monfresh]

I'll look into it, but I don't think a reset would have helped here. We have several specs that each depend on a specific service. For testing SMS, we want to use FakeSms, but for voice, we want to use FakeVoiceCall. The issue that caused the test to fail in Circle CI is that the test was expecting the FakeVoiceCall service, but because it was not set in the test, it used whatever was set by the last test, which happened to be FakeSms in this particular seed.