Thanks for replies so far.
Clearing carrier services data does not change anything.
Sending standard SMS is fine.
I am a new customer recently transferred from Vodafone. Chat was enabled on my old SIM when I transferred to VM. Additionally, on my first day at VM with temporary number, the Chat feature activated in the Messages app with the temporary number. When my Vodafone number transferred to VM, outgoing messages via Chat used the VM temporary number and not my transferred number. Now, the end result is that Chat now does not work full stop.
If this is Google's problem and VM cannot resolve, is there a method to get in contact with Google to resolve this?