There are three sorts of liaisons in French: mandatory, forbidden and optional.1
The liaison after est is mandatory and always pronounced:
- in the set phrase: c'est-à-dire /sɛtadiʁ/
- when the verb is followed by a subject pronoun: Est-il arrivé ? /ɛtil/
Other liaisons after est are optional but they are more or less followed according to the nature of the word that follows est.
Just frequent, often omitted in relaxed conversation, in most other cases, including in front of prepositions:
- Paris est en France /ɛtɑ̃/ or /ɛɑ̃/
- Il est à la maison /ɛta/ or /ɛa/
We note that the French language has been (and probably still is) operating a shift from mandatory to optional liaisons. For example the 10th edition of Le Bon Usage states that the liaisons between c'est and the following preposition, and between est and the following past participle are mandatory, but when we listen to people in France in the 21st century (I would not know about other French speaking countries and it might be different) there's a great tendency to drop it in casual speech.
- A guide to liaisons in French
- Quelques règles de liaisons