Telephone and online surveys both collect structured responses, but the best choice depends on how the audience can be reached and what the interview requires.
CATI can be useful when scheduled participation, interviewer guidance, or clarification matters. CAWI can be useful when eligible respondents can participate independently online and the questionnaire works well on their devices.
Start with four questions: Who needs to participate? How can their eligibility be checked? How complex is the questionnaire? What time and budget are available?
A combined approach may suit some studies, but differences between modes need to be considered in questionnaire design and analysis. There is no universal choice; the method should support the research objective.