[asterisk-biz] Is ISP Blocking VoIP

[J Thomas]

I provide hosted PBX service to businesses. About a dozen of them. All
of them were working great, but this morning suddenly, 3 places have
gone dead. None of the phones from those three places would register
with my server :-(

As the remaining 10 places are working just fine, hence I am reluctant
to blame this failure on Asterisk.

Changes is firewall setting at customer site could be a cause, but very
unlikely that three of them decided to do that at the same time.
Moreover, I specifically asked all three of them and none of them have
messed up with their routers.

Only thing common among them is that they all have business DSL from
Verizon. Could it be that, possibly inadvertently, Verizon is doing
something that is blocking VoIP? In any case, how to to find out why
VoIP is not working? 

In this case, all the clients have Polycom phones, about 30 among all
three.

Thanks,
-- jt