Make direct messages opt-out per followed

Make direct messages opt-out per followed, then only those friends and companies I select from those I follow can send me Direct Messages, each of which are texted to me. I have 3,400+ followers and currently I get an unwanted DM perhaps every other day. This is effectively spam, the unwanted DM is texted to me which has to then be deleted from my phone and from my Twitter account. Just because I follow someone does not mean I want a DM sent to hundreds of people, my interest may only be 'casual'.
Opt-in means people will use DMs more responsibly and be more considerate but will still give them at least one opportunity to send a DM. Some people have linked a mobile and request DMs to be sent as text messages, which costs money, so lets not waste it.