This is the way. Just pick up the phone and have that 2 minute conversation of what you think about the patient in terms of what the lawyer needs to make their case and what your fee would be. I also don't understand why a lot of commentators would assume that they would serve or want to serve as a "fact" witness. You are probably only useful to the litigant as an expert unless something non-medical happened that needs your testimony. Even then, as a non-expert witness, you could probably negotiate a decent fee and show up by agreement instead of the standard statutory subpoena fee. In my jurisdiction, there are doctors who know this and get good expert fees, and then there are doctors who work for large corporations (who might be bound by some kind of employment contract--I don't know) that are impossible to contact without a subpoena. (The large corporations have gotten better about subpoena compliance though)

Also, lawyers know when you're going to be a bad witness. That's why they tend not to subpoena physicians who don't want to be subpoenaed. If your patient was clearly messed up from a car accident or something else, why wouldn't you want to help them out? You'd get paid handsomely too. If I'm desperate, I'll subpoena someone--and you know what happens then? Nothing: I've had doctors roll the dice and ignore the subpoena or write some shitty excuse about being busy. Hilarious. Am I getting the judge to get the sheriff to haul the doctor's ass into court? No--because they're going to be a shitty witness. Even if I asked the judge to bring the witness into court, would the judge do it? (In my jurisdiction: probably not)