I ran a distributed team of 8, and was co-located with just one of them. It was easy enough to remain sighted on progress against deliverables via phone calls, emails, and a regular team video meeting, but extremely difficult to "take the temperature" of the team by such means. The best way to understand how individuals were feeling, or identify the undercurrents and interactions between members of the team and each other, or between team members and other workers in their locations, was to physically visit them.
Machiavelli wrote "In the beginning of the malady it is easy to cure but difficult to detect, but in the course of time, not having been either detected or treated in the beginning, it becomes easy to detect but difficult to cure." In other words, look out for the first sign of trouble and nip it in the bud. You need to have your radar switched on to identify even the slightest sign of an issue when talking to your team, and without the body language and other forms of non-verbal communications, this is not easy by phone. Pick up on every hint of a problem and decide whether it needs attention - don't leave it to chance.
Of course, this is very time consuming so you need to consider the best way to manage your time, and don't expect a PM with a distributed team to be as productive as a PM with a co-located team. The team may be as effective, but the PM is unlikely to be.
BTW - another definition of MBWA is Mismanaging by Wandering Aimlessly. Make sure it is productive!