I'm posting this on behalf of a friend. He bought an iPhone 7 and ported his number from Vodafone to Skinny Direct. When he tries to send a text message to Canada, it comes up with "not delivered". Resetting network settings didn't help. Skinny hasn't been able to find anything wrong, and suggested that he try the SIM in a different phone.

We've just put his SIM into my iPhone 6, and sent a message to the same number, and it sent successfully. We've been through the settings and can't find any obvious differences.

Texts to NZ and Australia work fine. Does anyone know what could cause this sort of issue?