Another possible alternative is to use email addresses. All carriers have email addresses tied to phone numbers. so you can simply use those email addresses for free to send/receive SMS messages if that works for you. Any phone can also sent an SMS to an email address, as opposed to a phone number, and as far as I know this has worked since years and years ago with every phone I've ever owned (even non-smart phones). This functionality just seems to be something mobile carriers all decided to include in their messaging systems.
You can find the list of carriers/email address domains here
Typically the email address is [10-digit number]@[providerdomain.com]