As per iOS SMS sending API, there is no option to send SMS programmatically without user consent. Developer needs to use the MessageUI framework to get iPhone user consent for sending SMS.
In that case, if any third party SmartWatch connected through BLE with iPhone received SMS notification through ANCS and want to reply to that SMS, After typing and sending from Watch, user needs to perform this additional step in iPhone - give consent.
But if we use Apple watch, this consent in iPhone is not required if Apple Watch is already paired with iPhone. After typing text in Apple Watch, can send SMS to receiver through utilizing iPhone's SMS service without any user interaction.
What is the reason of this difference?
For sending SMS, iPhone and Apple Watch needs to be paired. Similarly, even third party SmartWatch also performs BLE connection and pair together before sending SMS text from Watch to iPhone to forward to receiver.
But in that case why another additional user consent is required in iPhone? If we consider iPhone and Apple Watch case, pairing with each other is considered as user consent for sending any SMS later from Watch utilizing iPhone.
Then, why BLE pairing between iPhone and other third party Watch not considered as user consent and additional user consent is required for each time SMS sending?