@k8laka00 Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
If you want to use the Dynamic Alphanumeric Number in ACS resource. Could you please refer to the instructions shared in my Answer in the similar thread here and let me know if you have any further questions.
If you are looking for an Pre-registered Alphanumeric Number in ACS you need to submit an application by filling this form, Our engineering team will check and get back to you.
Refer to this documentation, to know more information/difference between Pre-registered and Dynamic Alpha numeric numbers.