I had a hard time figuring out how to access page tabs with mobile and, thanks to Ben in a previous comment, I found a way. First of all, you can't access page tabs with your mobile phone, so you will need to point it where your page tab takes it source. Facebook can do that for you if you are using canvas, but if you access it with a desktop computer, you will be redirected to your canvas link, not your page tab. This is how we fix it :
--
1) Go into your App (Facebook developers) under Settings - Basic
2) Copy your Page Tab URL from the Page Tab section (something like http://yourpagetab.yourdomain.com), and past it in your Mobile Site URL into the Mobile Web section.
3) Do the same with the App on Facebook section, but instead of pointing it into your page tab, you will have to use a redirection file. Let's use http://yourpagetab.yourdomain.com/redirection.php?fb
Note that you have to use ?fb at the end of your link. It does nothing, but Facebook wants it. Actually it could be ?anything and probably only ?
4) Create that redirection.php file (don't add the ?fb) and add this code in it
<script>top.location="http://facebook.com/your_fb_page/your_page_tab"</script>
You have tho replace /your_fb_page/your_page_tab with the link to your page tab.
5) Save it!
--
Now, all you have to do is to use the Canvas Page url that Facebook provides you (under App on Facebook). If people click on it and are on desktop, they will be redirected into the page tab. If they are on mobile, they will be redirected to your website hosting the page tab.
If you are using fan gate (user have to like your page before accessing your page tab) : this will not work as the mobile user will get stuck on your fan gate.
The only ways to get through that is to disable the fan gate or to use Mobile Detect (you can search it on Github) and disable it only for users that are on mobile. It's a small price to pay if you want your page tab to work with mobile users. Desktop users will still need to like your page to see your page tab.