What are the spam rules concerning SMS Marketing broadcasts?

Always provide an option for your recipients to unsubscribe.
If your recipient list is unsolicited, always check the list against the DNC registry at  http://www.pdpc.gov.sg 

For promo messages, do include a <ADV> tag at the beginning of your message.

See below for more info:

http://www.commzgate.com/news/commzgate_dnc_2014.html