If they make an offer over the phone (which I've had happen to me several times; it seems normal in the tech industry and probably also in startups), then I would thank them for the offer first, and be prepared to ask questions to get all of the data I needed (title, role, salary, benefits, travel, hours, etc.). I would then say that I would like a day or two to think it over and that I would get back with them.
Then I'd research and ruminate further as needed, then probably call them back with the next step, whether it be a declination, acceptance, or counteroffer. Counteroffer is probably the most likely.
They will certainly not expect you to accept or decline a verbal offer on the spot.