Transactional Text Message SMS Delivery Error (Invalid Phone Number)

All the AWS cloudwatch related SMS have stopped working in recent weeks. We just noticed it today. The Delivery status log says

delivery.providerResponse Invalid PhoneNumber

The number is a US number with format 1-nnn-nnn-nnnn. This format used to work in the past. Do we need to add a + sign in front? or there is an issue somewhere else?