The SIM could not to attach to the specified Network, either because its Fleet's Network Access Profile did not allow the Network or because Twilio has blocked the IMSI currently being used from attaching to the Network.
Possible Causes
The Network Access Profile associated with this SIM's Fleet has blocked access to this Network.
Twilio has disabled this Network for all SIMs.
Possible Solutions
Check the Network Access Profile associated with the SIM and enable the Network if you would like to use it.