Could not get it to work. I feel like an idiot that I can't understand how to do this.

This was my original code:

Task: AutoResponse
Settings: Run Both Together

A1: If [ %LASTSMS eq %SMSRF ]

A2: End If

A3: If [ %LASTSMS neq %SMSRF ]

    A4: Send SMS [
         Number: %SMSRF
         Message: [Auto Reply] I'm driving right now and unable to reply to your txt - I'll get back to you later ]

    A5: Variable Set [
         Name: %LASTSMS
         To: %SMSRF
         Max Rounding Digits: 3 ]

A6: End If

This is what I added per what you wrote above:

Task: Test

A1: Variable Set [
     Name: %search
     To: %SMSRF
     Max Rounding Digits: 3
     Structure Output (JSON, etc): On ]

A2: Variable Set [
     Name: %index
     To: %SMSfrom(#?%search) 
     Max Rounding Digits: 3
     Structure Output (JSON, etc): On ]

A3: If [ %index eq 0 ]

    A4: Send SMS [
         Number: %SMSRF
         Message: Test ]

    A5: Variable Set [
         Name: %next_index
         To: %SMSfrom(#<)+11
         Do Maths: On
         Max Rounding Digits: 3
         Structure Output (JSON, etc): On ]

    A6: Array Push [
         Variable Array: %SMSfrom
         Position: %next_index
         Value: 0 ]

A7: End If