The separate mobile site was an answer to early phones that could not render full pages, and businesses served a stripped down version from a different address. That approach carried real costs: two sets of content, two places to update a price, and search systems having to work out which version to show. Modern browsers removed the need entirely.
What replaced it is responsive design, where one set of pages adjusts its layout to the available width. The same content, the same address, and the same file being maintained once. Nearly every website builder and template produces this by default, which means most businesses already have it whether or not they chose it.
The reason to be clear about this is that separate mobile sites still exist on older builds, and businesses inheriting one frequently do not know. The signs are an address beginning with an m or containing mobile, content that differs between devices, and a phone version missing pages the desktop has. If you find that, consolidating is worth doing.
Being responsive is not the same as being good on a phone, which is the distinction that actually matters. A site that resizes without breaking still fails if the text is too small to read, the buttons are too close together to tap, the form requires precision, or what somebody came for sits below three scrolls of imagery. Those are design decisions rather than technical ones.
Test it properly rather than by narrowing your browser, which shows you the layout without the conditions. Open the site on your own phone, on mobile data rather than wireless, and try to complete the thing you want visitors to do. Find the phone number and call it. Fill in the contact form. Any step that makes you pause is losing people who will not persist as long as you just did.
Check what occupies the first screen before any scrolling, because that is what your site actually says to most visitors. If it is a large image and a menu icon while the answer sits further down, the design assumed a desktop even if it technically adapts.
Speed matters more on a phone than anywhere else, since the connection is worse and the patience is shorter. Images uploaded at far larger dimensions than they display at are the usual cause, and compressing them typically produces the largest single improvement available.
Then check the small things that only matter on a phone. Whether the phone number is a link that dials when tapped, and whether the address opens a map. Both are one line of code, both convert intent into action, and both are missing from a surprising number of otherwise competent sites.
Check what your site does at the awkward sizes as well as the obvious ones, since large phones and small tablets fall between the widths most templates are tested at. A layout that works at a typical phone size and a typical desktop can break in between, and that gap covers a meaningful share of devices.
Watch for content that appears only on one version, which is the residue of a separate mobile approach even in a responsive site. Elements hidden on small screens are still downloaded and they mean phone visitors receive a lesser version of your site while being the majority.