At some point business thought it would be helpful to have a mobile site for phone users. But then technology progressed and like many things, made them obsolete.
Since the monkey see, monkey do, nature of business everyone else had already made a mobile site, and since they spent money on it, decided to continue with it. Although we are reaching the point of we no longer need that.
Well I do not know much about mobile phones accessing mobile sites. But i did sometimes stumble upon a mobile version of some websites. The website checks what you are using to explore their site and sends you to the acquitted site. I noticed the url was different, typically mobile.Asite.com. So what i would do was remove the mobile or just make it say the original site name.
You can try playing around with the url, see if that works, maybe skipping the index page will prevent them from checking if you are using a mobile phone.