Anyone seen this kind of problem? SIP traffic not getting to destination but traceroute does

Wed Nov 9 18:47:37 UTC 2011

We ran into a strange situation yesterday that I am still trying to
figure out.  We have many VoIP customers but yesterday suddenly select
few of them couldn't reach the SIP provider's network from our
network.

I could traceroute to the SIP providers server from the affected
clients' IP just fine.  I confirmed that the SIP traffic was leaving
our network out the interface to the upstream provider and the SIP
provider says they couldn't see the SIP traffic come into their border
router.

SIP traffic coming from SIP provider to the affected customer came
through fine.  It's just Us -> SIP server was a problem.

I thought there may be some strange BGP issue going on but we had
other customers within the same /24 as the affected customers and they
were connecting fine.

The traffic at the time traversed

Our network -> Qwest/century link -> Level 3 -> SIP provider

I changed the routing around so it would go through our other
upstream, AT&T, and it started working.  With AT&T, the route was

Our network -> AT&T -> Level 3 -> SIP provider

So my questions is, is it possible there is some kind of filter at
Qwest or Level 3 that is dropping traffic only for udp 5060 for select
few IPs?  That's the only explanation I can come up with other than
the whole Juniper BGP issue 2 days ago left something in between in a
strange state?  I read the post about XO doing filtering on transit
traffic, I haven't seen anyone say Level 3 or Qwest is doing the same.