Almost every visit starts on a phone held in somebody's driveway
The design decisions that matter are the ones that survive a small screen, one bar of signal and a person who is not sitting down.
Design the phone view first and let the desktop version follow. Most roofing traffic is mobile, and the person is often outside, distracted, and doing this between other things.
A tap to call button belongs in the top bar and it belongs there on every scroll position, not just the top of the page. Plenty of your visitors have no intention of filling out a form.
Type has to be readable in daylight without pinching. Contrast, size, and buttons big enough for a thumb. Small gray text on white is a design choice that costs calls.
Nothing should move on its own. Sliders, autoplaying video and pop ups that fire before the page settles all steal the moment where the visitor was deciding whether to tap the phone number.
If a form is the primary conversion path, the first field should be visible without scrolling twice.