Que je vous ai appelés. -- That I called you.

You use the "s" at the end of the participle because of the vous referring to a group of people, hence plural. I think this concept is explained later on.

Here is the Kwiziq lesson on the subject -

https://french.kwiziq.com/revision/grammar/special-cases-when-the-past-participle-agrees-in-number-and-gender-when-used-with-avoir-in-le-passe-compose-conversational-past

The full answer is that a past participle that is preceded by the auxiliary “avoir” agrees in number and gender with the direct object if the direct object is placed before the verb (otherwise the participle is invariable). In this case, the past participle is “appelés”, the auxiliary is “ai” from the verb avoir, and the direct object is “vous” which is placed before the verb; therefore, the past participle agrees in number and gender with “vous” which is masculine (by default) and plural, and therefore requires an “s” at the end. It sounds more complicated than the previous answer but if one of these conditions did not apply, the past participle would not have necessarily taken an “s” at the end. For example, for the sentence “j’ai appelé les vétérinaires”, “appelé” does not take an “s” even though the auxiliary is from the verb “avoir” and the direct object is “les vétérinaires” which is plural, because the direct object in this case is placed after the verb. The rule is also different if the auxiliary is “être” instead of “avoir”. The rule is different again if the object is indirect rather than direct. Once you master all the rules then it is clear what the past participles’ endings should be, but until then it is not always obvious. Hope it helps (even if complicated)!