翻译：我想打电话回答她在上封信里提到的问题.

I want to call her to answer the question she asked in the last letter.
这应该是比较规范简洁的

I'd like to answer her questions that mentioned in the last letter on phone.

试一下：
I want to call her to answer her question in her last letter.

I want to call her to answer the questions that she asked in the last letter.

I want to take a photo and answer her mentioned in the letter.