For us, we use ISP provided SIP trunking that is technically unsupported. It was impractical for me to port all my numbers off of my existing provider only for SMS functionality. (Due to contractual commitments with Spectrum for our SIP trunking.)
Telnyx supports a solution called Hosted SMS, where only the SMS function of the your phone numbers is ported to their service. You then configure SMS messaging within your 3CX as directed in the guide.
This allowed me to keep my voice service unchanged, but also enable SMS functionality using my main trunk numbers.
There may be other supported SMS providers who also support Hosted SMS. But for me, it was a great solution and was super easy to setup.