You can find the answer to this question in Chapter 2 of Book the First, as Mr. Jarvis Lorry, the mysterious individual in the Dover mail who works for Tellson's Bank receives his message from Jerry Cruncher. It is important to note how Dickens shrouds this episode with dark, brooding mystery, and how he uses the setting to do this. Note how Dickens creates this scary location:
There was a steaming mist in all the hollows, and it had roamed in its forlornness up the hill, like an evil spirit seeking rest and finding none. A clammy and intensely cold mist, it made its slow way through the air in ripples that visibly followed and overspread one another, as the waves of an unwholesome sea might do.
Note how descriptions such as this combined with the deliberate way that Dickens does not explain the message or the answer both create a real mystery. Cruncher's message, therefore, is "Wait at Dover for Mam'selle." The response he receives, made more important by its capitalisation, is "RECALLED TO LIFE." As Jerry Cruncher himself comments, it is a "blazing strange answer," but we later find out in Chapter 4 that the message refers to the discovery of Dr. Manette, the father of Lucy Manette, the woman that Mr. Lorry is just about to meet in Dover. Having been jailed for a long period of time, he has been finally released, and therefore, metaphorically, "recalled to life."
Cruncher's message to Mr.Jarvis Lorry was,"Wait at Dover for mam'selle".
And the messanger was asked to return London with the message"Recalled to Life" by Mr.Jarvis Lorry in Book-1,Chapter-2 titled " The Mail" in Dickens's novel "A Tale of Two Cities".