@Dominik F. Remshardt As you are using function app version 1.x please refer to this document how to install the package that provides the Twilio bindings.
I will recommend you to use the function app 3.x and follow function binding twilio that covers the attributes/bindings and the example for usage.
As per the error message looks like the Twilio namespace was not found on your project i.e. the package was not installed correctly.