Dài zhe tǎntè de xīnqíng, tā bōtōng le nàge diànhuà hàomǎ.
Full of apprehension, he dialed the number.
Tips
memory
Both characters are a heart 心 with a direction on top: 忐 has 上 "up" and 忑 has 下 "down". A heart going up and down is an anxious one. The set phrase is 忐忑不安.