阝 on the RIGHT is the form of 邑 (city) — right-阝 always means "settlement, town, district." That's the indexing radical here: the outskirts of a city. Same right-阝 in 邮, 都, 部, 邪. Distinct from left-阝 mound (as in 陵, 阵).
交 supplies the sound jiāo directly, same tone. The "crossing" meaning has no role; pure phonetic. Same phonetic in 校, 较, 饺 — all sharing the -iāo/-iào rhyme. 交 here keeps its full six-stroke standalone form.
