I am not a teacher, but I am a native speaker.
I am assuming by "SMS", you mean "text message". SMS is the technical term that your average person does not understand. It could be:
"I sent an SMS to my friend."
But most natives would say:
"I sent a text message to my friend." or even "I sent a text to my friend". "Text" is slang for "text message", but it is gaining popularity quickly.
If this is not what you mean by "SMS", please let me know.
EDIT: Sorry, I missed the rest. Here it is:
"Sorry, something urgent came up. I don't think I can make it today. I will text message you as soon as it is resolved." Using "SMS", the last sentence would be: "I will send you an SMS as soon as it is resolved".
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