That's not unusual, and does not indicate that there's any kind of a problem.

A blank hop just means that responder is not configured to respond to requests where TTL=0. If the hop right after is fine, then you know the "non-responding" router is passing data down just fine.

We talk about this scenario in our VoIP troubleshooting guide:

http://www.pingplotter.com/manual/standard/voiptroubleshooting.html

Please let us know where this leaves you.

Best wishes,
Pete