You have attempted to add a Twilio phone number, alpha sender, or short code to your Messaging Service Number Pool, but you are already at the limit of phone numbers or alpha senders. By default, a Messaging Service can contain up to a total of 400 Twilio phone numbers and short codes and only one Alpha Sender per country.
Possible Causes
Your Messaging Service number pool is already full (default: 400 numbers)
You may have attempted to add more than one alpha sender for the same country to a Messaging Service or Sender Pool. Each country can only have one alpha sender assigned per Messaging Service Number Pool.
Possible Solutions
If you are sending to the United States and Canada, use a Toll-Free or short code number with higher message throughput (MPS), instead of adding more local numbers. Most mobile carriers in the US and Canada do not not permit Application-to-Person (A2P) messaging via long codes, so there is a high risk of carrier filtering when sending this type of traffic via long code.
If you are sending to other countries, you may be eligible to have your Number Pool size limit increased. Please submit a ticket with a description of your use case and why you need a higher number pool size limit. These requests will be reviewed by our Messaging team.