Hi and Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is
Welcome to Benefits and Work You can certainly ask that all communicating relating to your ESA claim is in writing. However, very few people will see your form and it is highly possible that someone will try to phone you if your number is on record. If this happens, you can say that you cannot deal with the issue by phone and ask them to write to you.
You might want to bookmark/favourite this on your web browser now so that you can find it easily in future. This will allow you to return with further questions/comments about your ESA claim without having to start a new topic each time. It helps us enormously when everything relating to the same claim is kept in one place - and I hope you will find it useful too!