I don`t have my 1099-R from Office of Personnel Management for 2016 and can`t get through to them either by phone or e-mail. Can I fill out the form myself?

Box 2a is the only box I am unsure of. How do you calculate it? Is it all right for me to fill out the 1099-R or does it have to come from OPM? Or, should I fill an extension until OPM sends me the form? I am hearing impaired. Please reply by  e-mail. (gagago@woh.rr.com)