I don't think there is a single word for waiting for a reply. One can expect, anticipate, await, envisage, assume, etc, but they all would need for a reply.
One can be expectant or expecting, but be aware that this word is commonly used for pregnant. One can also be anticipant, anticipative, ready, and hopeful, but I'm unsure these will meet your needs.
Expectant has a noble lineage.
- There was about an average of two dogs to one man; and these sat in expectant attitudes till a spent bone was flung to them, and then they went for it by brigades and divisions, with a rush... - Mark Twain
- ...and the crystal waters of the brooks, murmuring over the grey and white pebbles, hastened to pay their tribute to the expectant rivers... - Cervantes (Don Quixote)
- He saw the yellow road, the lines of expectant people, and the sober houses. Crane (Red Badge of Courage)
- the white birds were now all flying towards Ahab's boat; and when within a few yards began fluttering over the water there, wheeling round and round, with joyous, expectant cries. (Melville)