The Discount Phone line is for one number at a time. You can register multiple lines (multiple lines are discounted) but they must be set up with separate forward-to numbers.
https://www.inmateaid.com/ask-the-inmate/when-you-set-up-a-phone-line-with-inmateaid-does-it-only-let-the-inmate-call-to-one-number-or-can-he-call-more-than-one-number#answer
Accepted Answer Date Created: November 01,2019