this is legit - i had forwarded a message on my phone (voicemail) to my friends know about something being cancelled. the text they got said to call this number and there they heard the message i sent them.. nothing sinister here.
it's legit. A sprint customer that knows you, probably a friend, has sent you this message. So long as you call the 408 970 0000 number from the same phone that they "voice texted" you at, it will automatically play the caller's message (if they left a voice message along with the voice-text sms)