It can be beneficial to use more than one method of data collection. For example when a sample is drawn from the National registry there are no emails.
It's then common to mix both telephone- and online survey. A person is called and he or she is invited to participate by phone or get the survey sent via email.
To mix methods it's good to use the strength of each method. Mixing telephone- and online survey costs less than solely having a telephone survey.
It can even be good to mix together quantitative and qualitative methods, i.e. surveys and personal interview surveys. Personal interview surveys (qualitative method), e.g. personal interviews or focus groups give more depth and understanding of the topic.