There are a variety of synonyms that can be used to describe a "telephone firm." One option is "telecom company," which is a broad term that encompasses any business involved in providing telephone or telecommunications services. Another option is "phone service provider," which more specifically refers to a company that offers phone services to consumers or businesses. "Telecommunications corporation" is another option, which may be used to describe larger, more established companies in the telecommunications industry. Finally, "telephony company" is a term that can be used to specifically refer to a company that offers telephone services. Regardless of the specific term used, all of these synonyms are describing a company that provides some type of telephone-based communication service.