[Asterisk-Users] PSTN Incoming call on real line disrupts VoIP call over DSL circuit

[Hadar Pedhazur]

I haven't seen anything this strange, and it's 100% reproducible. I'm 
hoping that there are some clever ideas out there for what to look for, 
since I can test to my heart's desire on this one...

My Dad lives in Florida, and has a Bellsouth DSL line. Of course, he has 
a regular POTS line connected on the same line. He has the appropriate 
filters on every jack that has a phone connected to it, and he even 
replaced one or two of them (when I thought that was the problem).

I sent him a HandyTone GS-486 (HT), configured to connect back to my 
Asterisk server. He only has a single computer in his apartment, so it's 
connected into the HT, and the HT is connected into the DSL modem.

He can make and receive calls on the HT, and the quality is excellent. 
If he's speaking via the HT (meaning a VoIP-only call) and the "real" 
phone rings, everything continues fine (temporarily). If the real phone 
is answered, either by a person, or by the answering machine (which is 
in another room, connected to a filter on another jack), then the audio 
on the Asterisk conversation becomes _one way_. My father can be heard 
_perfectly_ by the remote side of the conversation, but he can hear 
nothing. When the POTS line is hung up, then both sides of the VoIP call 
go dead (audio-wise). Of course, he can now redial a VoIP call, and both 
sides work perfectly...

At first, I couldn't imagine that it was anything other than a bad 
filter, but other than replacing the filter (which didn't help), nothing 
else stops working. He can continue to use the Internet connection on 
his PC just fine, and I can continue to hear him speak over the VoIP 
connection with no problems either, so the Internet connection has not 
been lost.

I have to admit to being completely clueless as to what to even look 
for, so _any_ advice as to things to test for would be appreciated. As I 
said at the top, I can reproduce this 100% of the time, so I can easily 
setup any debugging environment in advance, and trigger the problem at 
will, etc.

Thanks in advance!