Typically they will have access to the Internet and the telephone at the same time and typically their Internet connection will be good enough to allow voice traffic. But again, there are no guarantees with voice quality on the Internet. Thus traditional voice is probably the best option. The customer can choose to be contacted on their landline and they type can in their number. They can also decide when to be contacted.
Most people choose immediate.
From here the request is sent to the call center where the availability of agents is checked. If there is someone available to take the call then the customer is given a nominal wait time of say 10 seconds to allow for call setup. If nobody is available then the customer can be given the current holding time, which they can accept, or decline in favor of a specific time. In any event, when it becomes time to contact the customer the customer will be dialed by the call button server and played a brief notification message before being connected to the call center. It makes sure that if a customer wants to talk to you then they do so.