el teléfono de casa (vs *de la casa)

Question

Hello.  Can you please explain why the definite article isn't used in this phrase?  Is it similar to the difference between "en casa" (at home) and "en la casa" (in the house, as in a physical location)?  Thank you!

Answer

Hola AA
it's common practice to drop the article with groups of words that form a set expression, phrase or definition of something, so, in this case we refer to the "home phone" (as opposed to our mobile phone) as "el teléfono de casa" - you can of course use the article too and the meaning is still the same, but there is no need here because it's used as that set phrase. the "de casa" is descriptive of the noun teléfono. We use de + noun for description when in English you express that with a noun in front, see here. 
En casa is again part of set expressions like "Estoy en casa" (I'm at home), or "Hoy me quedo en casa" (Today I'm staying at home.) or "¿Estás solo en casa?" (Are you home alone?)
Saludos