The rules of past participle agreement is a hot topic.
In your cases, the rule is simple: the past participle used with the auxiliary verb avoir agree with its direct object if the direct object is located before the verb.
However, your first sentence has an extra difficulty. It might be written either:
Elle l'a appelée.
Elle l'a appelé.
Here the direct object is l'. Some context is required to know if this object pronoun with an elided vowel refers to a feminine (la) or masculine (le) antecedent.
As appelée is used in you example, the antecedent is likely a woman:
Elle l'a appelée (She called her)
Should the pronoun had referred to a man, that would have been:
Elle l'a appelé (She called him)
As there is no neutral in French, She called it translation would use either appelée or appelé depending on the gender of the word behind it.
There is no issue with the second sentence, the direct object is after the verb, there is no agreement and the masculine is used:
Elle a ouvert la porte
Elle l'a ouverte