SMS not delivered if receiver offline at sending time

[alpineduck]

Hello,

I've been using the PinePhone regularly for about half a year. Calls and SMS have been reliable, with the notable exception of the following problem that I would like to understand, and hopefully solve:

If I send a SMS to a phone that's offline, the expected behaviour of any SMS service is that the message is temporarily stored by the carrier, and when the receiver phone is later brought online, it gets the message delivered. For some reason, this isn't the case for the messages I send from the PinePhone. Messages are delivered properly when the receiver is online, but they never reach their destination if it was offline at the time of sending.

I tried with two different carriers and both 3G and 4G networks for each: the problem is systematic with the PinePhone, while messages are delivered properly when the same SIM card is used in another phone. The problem also persisted across various upgrades of the modem firmware, including ADSP.

When logged in to my user account on my carrier's website, I can see that the messages were successfully sent.

To me, this looks like the PinePhone sends its messages together with some kind of metadata meaning "do not attempt later delivery, instead discard the message if the receiver is unreachable". Could anyone confirm this? Can it be changed and how? I couldn't find any information on that matter and to be honest I'm quite surprised to be the first one to bring this up.