You need to interact with Toby in every case to pop this achievement/trophy.
In "The Fate of Black Peter," Sherlock will say this..: "Brave toby, the best nose in the british empire."
In "Riddles on the Rails," he'll say this: "Look after Mrs. Hudson for us, Toby."
In "Blood Bath," he'll say this: "On the sofa for the third time today. Well, don't be surprised if Watson sits on you again."
In "The Abbey Grange Affair," he'll say this: "I agree with you, Toby, that Watson's shoe is preferable to Mrs. Hudson's cold cuts.
In "The Kew Gardens Drama," he'll say this: "Oh dear, Toby. I wasn't aware that you were Watson's new literary agent."
And in "A Half Moon Walk," he'll say his last phrase and you'll unlock this achievement/trophy, which is this: "Hello, Toby. All you need now is a shawl and a mop cap, and you could be Mrs. Hudson's younger sister."