If you have canceled a non-refundable United e-ticket ticket, you can use the ticket value (minus any applicable change fee) toward a new ticket via phone, naturally without having to mail anything in, since you have nothing to be mailed. Ideally you know the United 016 ticket number; sometimes knowing the PNR (record locator) is sufficient, but don't count on that, since PNR's do eventually get reused. Keep track of your 016 numbers!
If there is any residual value leftover in such a ticketing transaction, then that residual value gets printed out into Type B voucher form, and to make use that value in the future, you would either have to go to the airport; or call United to make the reservation (the phone agent needs to mark your reservation as one which is pending ticket-by-mail processing) and then mail the voucher to the ticket-by-mail postal address in Dearborn, MI.
YMMV when it comes to whether the United agent you're working with is able and willing to let you combine as many vouchers/certificates/discounts as the rules will allow. I've usually been successful in combining several Type B vouchers (forms of payment) plus one Type A voucher (discount).