According to Globes, the Israeli Ministry of Communications has instructed Internet service providers (ISPs) to ensure that their systems do not carry international calls over VoIP. However, the effect of this directive will likely simply favor the use of peer-to-peer voip networks such as Skype which were not included within the scope of the directive even though many now also offer PSTN termination (such as SkypeOut).
Between these types of announcements and the problems some people seem to have with their local ISPs attempting to block VOIP calls, a good solution may be to build in more flexibility to use non-standard port numbers for VOIP traffic. That would likely make VOIP more difficult to block as a service.
Update: There are some reports that this action may be related to cutting down on the selling of cheap international phone cards. As Jeff Pulver notes, a number of Israeli companies were among the pioneers in the development of VOIP and Israel generally has inexpensive international call rates.