If you're not meeting your clients in person, then your options are limited: phone or videoconferencing.
The problem with videoconferencing is the competing standards. There's (deep breath): Skype, Facebook, FaceTime, Google Hangouts, WebEx, and more. Some are mobile-friendly, some are not. Some are great for Apple users, some are better for Windows users.
Your best bet is to be familiar with all the standards, just in case your client is a die-hard adherent to single platform. And if not, there is an up-and-coming cross-platform standard that is compatible with nearly everything.