Hello, 

I understand that some governments are considering to ask all their ISP to
close the VOIP port so as to prevent IP telephony. By having this port
closed, would they also indirectly prevent the use of voice-activated
Instant Messaging (IM) ?

Thanks

That would depend on which ports the VoIP service uses and the voice
IM.  If they are the exact ports, then yes, it would be blocked.

As an FYI, this company http://www.vonage-promotion.com has plans to
rollout world wide VoIP services later this year, so I do not know how
that would be effected.

This is a good one...  Close the VoIP port.  What VoIP port?  Internet
telephony can happen on any of the 65,000+ ports available.  There are
some standard ones that are commonly used, but they can easily be
changed.  VoIP (e.g., SIP/RTP) can also tunnel ( e.g., on port 80
(http) ) so there is no real way to stop it.  The real worry is the
FCC/USGOV making VoIP telephony "illegal" for an ISP to transport with
out being measured, regulated, or taxed.  It all roots back to the
Communications Act of 1934 (Universal Service), the MFJ of 1982 (Bell
System Divestiture), and the Telecommunications Act of 1996 (the
Telecom Implosion Act for the new millennium).  I digress.