I have had an annoying problem for almost a year. Whenever someone picks up our phone, the DSL modem would lose sync for a minute. Usually that was enough for some connections to time out. Since we don’t use the home phone much, I put up with this longer than I should have.

I called AT&T to have them check out the line. It passed their automated line test. Before this, I had carefully narrowed down the problem. I unplugged all phones from their jacks and made sure each had a proper DSL filter on them. I checked the alarm system. I tried with a different phone to be sure it wasn’t that. I moved the DSL modem to another jack. No difference. Picking up the phone or going back on hook would cause the modem to lose sync. At all other times, it was fine.

The tech came out and did some line quality tests. We disconnected the internal wiring and plugged the DSL modem directly into the external wiring. The problem still happened. He called for some assistance but his support was baffled too. He finally apologized and said maybe the modem was bad.

Last night, I tried with a different modem and had the same issue. I did some more looking and found a bit of information on this. Back in the old days, Pac Bell would install an MTU (maintenance test unit) or “half ringer”. This device allowed them to do a line test without the customer being involved. However, the voltage change of going on-hook causes it to “bounce” the line. Before DSL, this didn’t matter because no one was on the line to hear the bounce. DSL is like an always-on modem connection so any noise or interruption will cause it to restart the sync cycle and you lose your Internet for a minute.

I dug into my telco box (NID) this morning and found this was the problem. To prevent others from wasting hours arguing with phone support that there really is a line problem, here’s how to diagnose this yourself. I’ll use my box as an example, but keep in mind these devices come in various shapes.

First, find your telco box. This is where wires enter from the street and connections are made to your inside wiring. There’s a screw on the right that allows you to open the cover.

Once you open the cover, you’ll see two sections. The inside wiring is on the right and is accessible by opening each terminal cover. The telco side uses a special screw so it’s harder for you to open. In most cases, you won’t need to open that side anyway. As you can see, only the top two terminals of my box are in use for inside wiring. The others are still available. If removing an MTU, you only need to do it from lines that are actually used. I found that every single one of these terminals had an MTU behind it!

Just to be thorough, I checked inside AT&T’s side of the terminals. Indeed there is no MTU here, just some wiring posts.

The MTU is the little black circuit board here, behind the terminals. It is wired in series with the inside wiring so I can’t just cut it out. Some people cut it out and then use gel-filled wire nuts to splice the wires. I chose an easier and less clean route of stripping the wires and attaching them directly to the screws on the right side.

I repeated this for both terminals that were in use. I didn’t bother with the others for now. Finally, I put everything back together and tested for dial tone. DSL was working and the problem was gone!

Here are some other links to info about this problem and pictures of other MTU devices.

• FAQ: My DSL disconnects when I answer the phone
• DSL loses sync when phone is used
• ADSL drops when phone goes off hook
• Another type of MTU and another

All in all, this wasted about 6 hours of my time troubleshooting, calling AT&T, explaining it to the tech, etc. Too bad I can’t bill them for my time. I hope this article will save your time and that the telcos will educate their support staff more on this very common problem.