Twilio: $DIAL + phone number

[dmarvp]

Is your feature request related to a problem? Please describe.
Currently, there's a global phone number that is set as part of a configuration. This prevents us from doing tests against different phone numbers from different tests.

Describe the solution you'd like
I would like to be able to set the phone number also per test. For this, the instruction would look like this:

- test: Phone number on test
- $DIAL 202-555-0161: Hello how can I help you

Additional context
We'll keep using a global phone number (should be the default if no number is provided on a test), but this gives us more flexibility and it's also easier to read/understand.