For Graphene OS? phones, KaiOS phones, and all other basic phones, the only option for IM? communications is SMS?. SMS uses the voice channel for communications as this technology preceded data services on cellular phones. There had to be a way to send a message with using existing technology, and use the voice channel was the only option. If you were to send an SMS message to me, you would want to check with your carrier to make sure the SMS message is affordable. If you are using most smartphones, you would likely be using RCS, or iMessages – and this is not an issue for you.