If you see a message saying the phone number is warming up and not able to text yet, that means the number is in it's activation period. In general, activation finishes automatically within 3 business days of adding the number to your account.
During this activation time, outbound texts cannot be sent to US phone numbers. You can receive texts, and make and receive phone calls during this time.
Why do new phone numbers need to warm up?
U.S. mobile carriers now require business phone numbers to be registered to send text messages. Mobile carriers now require that we request outbound texting activation for each new phone number. We are working with our texting partners to shorten this warm-up period for new numbers so you're able to start texting faster.