The reference letter and employment confirmation letter should do, but try to attach as many formal documents to corroborate the contents of the reference letter as possible. They wouldn't call your employer. If at all, they may call the colleague who provided the reference letter. In my case, I didn't even ask for the reference letter from my employer as I knew I won't get. I got a reference letter from one of my ex-managers on a plain paper along with his business card. In addition to this, I provided a whole lot of documents to establish the contents of the letter; my latest salary hike letter, all promotion letters and offer letter. Luckily, my employer is in the habit of providing formal letters for everything, which came handy. I feel it is better to get the reference letter from your current or past manager and attach at least one formal letter on company letterhead with his/her signature to establish that he/she is/was indeed your manager. I wrote a detailed explanation letter as to why I didn't provide reference letter in company's letterhead and explained all that I have provided. Got my application approved without a question or a phone call
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However, this is only my case and will vary on case-to-case basis depending on what documents you submit and how receptive the visa officer is
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