why?

What you overheard (past tense) was someone calling her (present tense) Ashley.
“I overheard someone who called you Ashley.” This sounds incomplete: the phrase “who called you Ashley” becomes parenthetic, and the remainder of the overheard conversation is called into question.

habibaelgindy Why do I say "I overheard someone call you ashley" Why can't I say called you?

Verbs of perception (see, hear, overhear, ...) all form catenative structures with the following verb. This second verb must be in its dictionary form (no ending like -ed) or in its -ing form.

I saw someone take the money. (Not

You see/hear/etc people do (bare infinitive) or doing (-ing form) things.

fivejedjonYou see/hear/etc people do (bare infinitive) or doing (-ing form) things.

Oops. I'll change that.

CJ

Can I say I overheard someone callS you ashley?

You might know someone as “Ash” but overhear someone calling her “Ashley”. Then you might say, “I overheard that someone calls you ‘Ashley’.” However, “I overheard someone calling you ‘Ashley’.” is the more common.