It may be possible for you to send a mail instrument to a nationwide sample. You may be able to conduct phone interviews with them. But it will almost certainly be less feasible to do research that requires interviewers to visit directly with respondents if they are widely dispersed. The sample is the actual group you will have to contact in some way. There are several important sampling issues you need to consider when doing survey research.
Some people are very busy.
Some travel a lot. Some work the night shift. Even if you have an accurate phone or address, you may not be able to locate or make contact with your sample. Let's say you draw a sample of households in a small city. A household is not a respondent.