Email is still effective, even though it's the most cost-effective way of sending spam. This is due to spam filters.
On Android for instance call blockers are allowed on the marketplace (versus iOS) - https://market.android.com/search?q=calls+blocker&c=apps - many of those apps allow you to have a blacklist of phone numbers, or you can just specify that you only want SMS from your contacts.
Better filters can always be built. Also, even though the sending of SMS may be free at some point, getting new numbers will not be free and getting your numbers flagged by blacklists will get costly.
On Android for instance call blockers are allowed on the marketplace (versus iOS) - https://market.android.com/search?q=calls+blocker&c=apps - many of those apps allow you to have a blacklist of phone numbers, or you can just specify that you only want SMS from your contacts.
Better filters can always be built. Also, even though the sending of SMS may be free at some point, getting new numbers will not be free and getting your numbers flagged by blacklists will get costly.