Another interpretation: "The voice is the voice of Jacob" (Genesis 27:22) - you have no prayer that is heard [accepted] which does not have in it some of the seed of Jacob.
No Prayer Is Accepted Unless the Seed of Jacob Is in It
Yalkut Shimoni on Torah 115:9
דָּבָר אַחֵר הַקֹּל קוֹל יַעֲקֹב, אֵין לְךָ תְּפִלָּה נִשְׁמַעַת שֶׁאֵין בָּהּ מִזַּרְעוֹ שֶׁל יַעֲקֹב.