Look, if there's one thing Covid's taught us... everyone hates video meetings. They're awful, somehow more so than the regular sort, and anyone who says otherwise is playing Minecraft with the video turned off and their microphone muted.
Demons are no different. It might even be that they are tortured in Hell with constant video meetings. (Side note: design experiment to test whether I'm actually in Hell.)
So, he's just gotta meet face to face. Good salesman will tell you that they do most of their business with a handshake. I'm not suggesting the wizard do that literally, but for a bargain of this magnitude it has to happen. They have to meet physically. No self-respecting demon is going to give Bob the time of day unless he's willing to sit down at a table (or stand at a crossroads) and make his pitch. It may be technically true that the demon is the one selling the service here, but like any good tradesman, the demon has more than enough customers, while Bob only has the one demon who will sell him this service (a classic monopoly). This wizard has to cater to the demon, not the other way around.
If you were selling something that's in-demand, so much that you have no free time, are you going to go out of your way for someone who places a telephone order?
It wouldn't be bad if they got to know each other. To appreciate what they have in common, their desire for the utter despair of the Abyss. Their soul-deadening diabolical nature, their yearning to crush all hope. To see that this is much more than just a business deal, but almost a favor from one friend to another. This is how deals were done in an earlier time. This is the sort of deal Bob needs to strive to make. He needs to put on his best clothes, head over to the desolate crossroads at the witching hour, and to make sure that he has a pen inked with his own blood ready to go (if you have to ask the other guy for his, are you even serious about the Faustian contract?!). He needs to look straight ahead with shoulders tall and straight, not shuffle to the meeting with eyes downcast.
Any Wall Street hedge fund manager will tell you this is how deals with devils. There are like at least a dozen books on the shelves at Barnes and Noble that lay it out in better detail than I can. Though, I thought this site was supposed to be about fiction questions.