Occasionally, given geographic distance, the phone will be your natural choice.

However, frequently you will network with people in your geographic proximity and you may need to choose between a phone or an in-person meeting. Essentially, your preparation will be the same. What is different that it may be easier to connect if you are face-to-face with someone. At the same token, having lunch or an in-person meeting with an individual where there is very little chemistry can be challenging, to say the least. Sometimes networking can turn into agonizing.  My recommendation: the phone can be very effective for general networking purposes and you can always practice in-person interactions at greater events such as conventions or company presentations. Your personal style, preferences and geographical opportunities will likely play a significant role in which approach you decide to use.