The question about what constitute "therapy" is no different than when a client calls you to make arrangements etc; that is not therapy and as a trained practitioner would never engage in therapy in an unscheduled telephone conversation. This is the same for any online therapy.
Finally everything resides within the ethical frameworks of the regulatory bodies - for example, not working outside the practitioner's limit of competence. To be unclear about the differences described above and engaging somehow in therapy as a result would show a limit to the practitioner's competence.
SMS, mobile phone to mobile phone should NEVER be used for therapeutic purposes. Email and IM/live chat should only use GDPR compliant platforms for therapeutic purposes.