Delivery report "unsuccessful"

[pete5000]

So usually when I text people, the delivery report under Show message details shows Requested until it's received on the other persons phone, at which point it changes to Received.

Today I messaged a friend, and the delivery report status is immediately showing as "Unsuccessful" but there is no explanation of what exactly this means. Should I send it again? I don't want them to receive this message twice. I have messaged this person many times before and delivery reports are always accurate.

The reports are still coming up as requested and then received when I text other numbers.

Does anyone know exactly what the "Unsuccessful" status means and whether or not the message could still have been delivered or not.

Extra details: In the android sms database the sms status column has the code 65.

Device: Samsung S7 Edge 7.0 Nougat
Carrier: BT Mobile (UK)
Thank you

 

[ManiacJoe]

The unsuccessful delivery could be as simple as their phone is turned off or they are getting no cel signals.
The code 65 could be interesting.

 

[pete5000]

The unsuccessful delivery could be as simple as their phone is turned off or they are getting no cel signals.
The code 65 could be interesting.

If a phone is turned off or doesn't currently have signal it the delivery report will stay as "Requested" until it is received on the recipients phone. From what I could find code 65 is the Failed status, although it shows the status as Sent on the message details.
I just have been unable to find a definitive answer anywhere as to what exactly "Unsuccessful" means in the Delivery report and if this categorically means that the recipient did not receive the message.