Another plus of the Voiceover CD is the availability of teachers, who can be easily reached by means of a telephone, but this kind of service is expensive.

by 在这里表示"通过某种方式或手段"。 means 是名词，指"方法、手段"，单复数同形。 of 在这里表示所属关系，连接前面的名词和后面的具体工具。 by means of 是一个固定短语，意思是"通过...的方式/手段"，后面接具体的工具或方法。