If a player asked me to come up with a warlock backstory for them, I'd veto the character and make them play something else. Warlocks have awesome backstories built into their class, and the extent of their pact (how much service they have to give their patron, how they fit into their patron's plans, how they think they fit into their patron's plans...) is possibly the single most important factor in how that character would think and act. IMO it's like a fighter asking the DM to choose which weapon they use, or a cleric asking the DM which domain they want to play.
If you're cool with choosing such an important part of a PC for them, more power to you. Typically warlocks make pacts with archdevils (and occasionally pit fiends), while lesser devils like imps might be used as intermediaries. Depending on the patron, a telepathic link might be made such that direct communication -- either in the form of visions/dreams or a telepathic communication -- would be the preferred method of communication.
You could always make the character think he outmaneuvered his patron and got an amazing deal, when in reality he got shafted and is given a trivial amount of power in exchange for an enormous commitment (including his soul). Or the warlock could be willing to forfeit his soul as long as his thirst for blood is slaked, or he could have accidentally made the deal, agreeing to something which he didn't understand at all (only that he was told he'd receive a fate beyond his wildest dreams). Good-aligned warlocks can also be interesting: he makes a deal with a devil to have the power to stop a greater, or at least more immediate, threat. Once that's done he realizes there's an even bigger evil which he can't fight by himself, and enters into another contract, and it keeps escalating from there.