Short answer: measure width at the top, middle, and bottom of the opening, and height at both sides, since almost no shower opening is perfectly square. This alone will make any phone quote far more accurate.
What to measure
Width: measure the opening in three places, since tile and studs are rarely perfectly plumb. Height: measure both the left and right side, since the shower base or tile ledge is often slightly uneven.
What you can't measure yourself
Wall plumbness and floor level need a real level and a professional eye — this is why a phone estimate is always a range, and why final pricing waits for an in-person measurement before glass gets ordered.
Why this matters: frameless glass is cut to your exact opening with almost no room for error, so an inaccurate measurement at the quoting stage can mean a real difference in final price once it's remeasured properly.