诶 has multiple tones depending on meaning: éi (surprise/calling attention), ēi (agreement), èi (reluctant agreement), ěi (disagreement). The 2nd tone éi is most common.
Two-stroke speech radical on the left - the simplified side-form of 言. Indexes 诶 in the talking family with 说, 话, 谁, 请. Marks this as a noise that comes out of the mouth: an interjection, exactly the role 诶 plays in conversation.
Right side 矣 supplies the sound - a tone shift away from yǐ. 矣 is itself a classical sentence-final particle ('and that's that'), so it adds a faint flavour of conversational closure. Pair the speech radical with this sigh-particle and you get the casual 'eh / hey' attention-grabber 诶.
