[DOC] - Note messaging capability being disabled on new US 10DLC purchases

[joe-conrad]

Describe the requested update
We need a call-out or blurb to note that newly purchased US 10DLC numbers do not have messaging capabilities enabled by default. Messaging is automatically enabled once the number is assigned to an approved TCR campaign. For inbound-only use cases, customers can contact support (Carrier Ops handles) to have messaging enabled specifically for inbound-only.

Reason for the request
Beginning December 15, 2025, messaging is disabled on all new US 10DLC purchases by default. Numbers purchased in support of an inbound-only use cases now require some vetting and specific enablement before being able to receive messages. Any numbers used for mixed use (in/out) are automatically enabled once assigned to an approved TCR campaign. Similarly, the first US 10DLC number purchased in a space that gets paired with a verified number for Platform Free Trial (messaging testing) is also enabled automatically for that purpose.
Support seems to be fielding many questions about this from newly signed up customers (and some existing). While comms were sent to existing customers, we should add something to docs (Phone Purchasing? Messaging related pages?) noting this.