Send as chat message

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Wondering why this feature is not always available. I have a S9+ in Australia using Telstra. 

Texting some of my contacts and would like to be able to turn this on especially to be able to share videos  and when I use the three dots in top right not all have this option.

Have also noticed that sometimes in others where I have been able to use it, it has changed by itself back to sms messaging and have had to go in and turn it back on to chat