וַיִּגַּ֧שׁ יַעֲקֹ֛ב אֶל־יִצְחָ֥ק אָבִ֖יו וַיְמֻשֵּׁ֑הוּ וַיֹּ֗אמֶר הַקֹּל֙ ק֣וֹל יַעֲקֹ֔ב וְהַיָּדַ֖יִם יְדֵ֥י עֵשָֽׂו׃
האָט יעקב גענענט צו זײַן פֿאָטער יצחקן, און ער האָט אים אָנגעטאַפּט, און האָט געזאָגט: דער קָול איז דער קָול פֿון יעקבן, אָבער די הענט זײַנען די הענט פֿון עֵשָׂון.
Rashi
THE VOICE OF JACOB, because he speaks in an entreating strain —“Arise I pray thee.” Esau, however, spoke in a harsh strain (v. 31) “Let my father arise” (Midrash Tanchuma, Toldot 11).
Rashbam
הקול קול יעקב, seeing that Esau and Yaakov were twins, their voices were similar to one another. This made it easy for Yitzchok to err as to who was in front of him if he were to rely only on his sense of hearing. Having first established that unlike Yaakov who was smooth skinned, the son in front of him was hairy, Yitzchok was now faced with a dilemma whether to trust his sense of hearing or his sense of touch.
Radak
ויגש...והידים, he did not examine the skin on the (supposed) smooth part of his throat. He relied solely on checking his arms.