"The girl was answering the phone."
Translation:La fille répondait au téléphone.
"Au" is the contraction of "à" + "le".
La fille répondait au (à+le) téléphone.
I am not understanding why the "a" is needed though? Can you direct me to a link where this is explained?
The only thing you have to understand is that prepositions need to be learned as part of each verb's construction.
- to answer sb/stg (transitive) = répondre à quelqu'un/quelque chose, indirectly transitive with the preposition "à".