Every new arrival at a federal facility must complete an orientation period before any privileges are granted. This includes phone access, visitation, and in some cases, commissary. The orientation process at most BOP facilities, including Texarkana, typically takes about a week, sometimes a few days less and occasionally a bit longer, depending on intake volume and scheduling.
During that window, your person is getting processed, classified, briefed on facility rules, and assigned to their housing unit and work detail. Phone lists, approved visitor lists, and commissary accounts all get set up as part of that process rather than before it.
The good news is that incoming mail is generally received during orientation, even before outgoing communication is set up. Sending a letter immediately after you know they have arrived is a good move. Include your phone number clearly so that when their phone access opens up, they have what they need to call you right away.