Oppo
SMS Delivery Reports: Turn On or Off auf Oppo
Delivery reports send a short confirmation to your Messages app once a text has reached the recipient's device. On Oppo phones running ColorOS, this function is turned off by default, so every SMS you send simply disappears into the thread without any status update.
The setting lives inside the messaging app itself, not in the general system settings, which is why it is easy to overlook. On dual SIM devices, ColorOS treats each SIM slot separately, so enabling the report for one card does not automatically activate it for the other.
To turn it on, open Messages ➔ Menu ➔ Settings, then pick the SIM card you want to configure if you use two SIMs. From there, toggle Get SMS delivery reports on or off.
Keep in mind that a missing delivery report does not always mean the message failed. Some carriers or recipient networks do not send confirmation signals back, so the report can stay blank even though the text arrived. If you need proof that a message was read rather than just delivered, this setting will not help, since ColorOS only reports network delivery, not read status.
Turning the feature on for both SIMs has no real downside beyond an extra status line in each conversation, so it is safe to enable on both cards if you regularly switch between them.