It has to be 16 digit. Yours is 15 digits. Just add 0 (zero) in front. Example, if yours is 35xxxxxxxxxxxxx (15 digits), just add 0 like in 035xxxxxxxxxxxxx (16 digits).
At end of every trial, just change the number to any other number (16 digits). Example, for this month use, 035xxxxxxxxxxxx1, next month use 035xxxxxxxxxxxx2, the upper month use 035xxxxxxxxxxxx3, etc.
NOTE: xxxx means any number.