[jblwps]

Assuming you're talking about the Android app, but you can actually force sending with SMS. The option to do so in a conversation/thread can be found by long-holding the send button, which then pops up a context menu to send via Signal or SMS.

[stilisstuk]

This is not a long term solution :)

[rvense]

It's a per-conversation preference, as far as I remember. You only have to do it once.

[stilisstuk]

In that case: Thanks a lot! Did not know.