I need a help desk tool which support all types of CMS and also supports mobile application such as iPhone and Android.
I would recommend using ZenDesk. It's featured in an article just published, 4 Android Apps You Need to Manage Your Own Business. Freshdesk is another similar help desk tool.
If you're using WordPress, there is probably a helpdesk plugin available for that, but I don't think they would have iPhone and Android integration. Maybe something paid-for would.