The correct VoIP equipment doesn't use that much bandwidth. Heck we still have a few dozen dialup customers some of whom successfully use Skype over their dialup internet connections here in the greater Ajijic-Chapala area at (www.laguna.com.mx)
So, yes, if you have limited bandwidth and are using a bandwidth hog like Vonage, you might have a problem when multi-tasking. But with appropriate bandwidth, both up and down, you won't have a problem.
PS I don't know the bandwidth requirements for Magic Jack but I can not perceive them to be more than Vonage and there is sufficient bandwidth available here for what you were wanting to do, much if not most of the time. Shown here are the speeds offered for example cybercable; a division of www.telecable.net.mx You should get perhaps 80% of these "up to" speeds much if not most of the time