A proposal is built to win. By the time one reaches your desk it has been shaped and rehearsed by people whose job is to make the work look inevitable, and most of what would help you decide has been polished off along the way. You can read three of them in a row and still not know what usually decides whether a project goes well, which is how a studio thinks before anyone has prepared the answer.
That is what a first call is for. Fifteen minutes with the people who will run the work tells you more than fifty pages of proposal, because a document is rehearsal and a conversation is much harder to fake. What matters is knowing what to listen for.
How a studio behaves off the script
Where a proposal shows you what a studio wants you to see, a call shows how it thinks once the script runs out. So watch the order of things. A studio worth hiring asks about your business before it talks about its own, and somewhere in the first few minutes it pushes back on something you said, because it is already working the problem instead of waiting to be hired to start. The ones to be wary of agree with all of it and write it down for the proposal.
What to listen for
The useful signals are the quiet ones. A good studio reaches for your actual problem rather than the nearest case study that resembles it, and it gets specific fast instead of staying safe in language that would fit any client. It matters, too, who is on the call, because the people assembled to win the work are not always the ones who will do it. But the signal that tells you most is the simplest. Whether a studio can say plainly what it is not the right choice for. A team that knows where it is weak has usually learned it the hard way, and a team that claims none has never reached them.
A good call should make it easy to say no
The point of the conversation is not to close you but to find out whether the work is a fit, and that fit runs in both directions. A studio sure of its work will tell you when you would be better served elsewhere, because the wrong project costs it far more than the lost fee ever would. So the best thing that can happen on a first call is that nobody is selling. You come away knowing whether to keep talking, and so do they.
Save the proposal for when there is something real to propose. Start with the call, and judge a studio on the questions it asks rather than the pages it sends.