First I am a telephone guy, I dont do security or alarms.
I have a customer that we just put in a new Toshiba and T1 with 6 digital lines out of an Adtran unit. Four lines go on the system, one for fax, and the other for alarm system. The alarm pad at the front flashes & chrips every 20 seconds, it did this before on the local copper lines too. The vendor stated the line shows a voltage drop is causing it and claim this issue is not a problem with the equipment.
Like I said Im a telephone guy but this seems like a equipment problem, they have no problems on the voice lines or faxing and nothing else is using the security line and we have switched lines that it uses and the problem still exists.

Anyone have any thoughts or suggestions?

Check the on-hook voltage from the IAD. It may be lower than 48VDC, and the alarm is interpreting that as off hook without current flowing through the alarm circuitry. The alarm installer should be able to change settings on his side to compensate.

Not sure about the Adtran IAD's but on the Cisco IAD's you can change the "idle line voltage". The service provider would need to change it.

That means that any discussion about changing voltages from channel banks is moot.

That means that there is something amiss in the alarm panel.

Have the alarm installer meet you at the site, wire in a known-working POTS line that has 48 volts on it, and see what happens.

Is the alarm panel programmed to expect to see 2 dial tones? Most commercial systems want both lines to be powered in order to operate correctly. Since you indicate that there is only one line (which is a violation, of course) try installing a second POTS line (or even a line from the channel bank) and see what happens. Or put the same dial tone on both RJ31X jacks just as a test.

Please keep us posted.

I will be going to site with or without alarm vendor who seems reluctant to meet. He claims that with a meter on either of the two lines that his equipment has that every 20 seconds the voltage drops to 7 volts? There is nothing else on these two lines except the Adtran 908e unit. He has placed DSL line filters in line on both lines as well according to the site contact. He has stopped the repetative beep to the entry pad by extending a timer? to 45 seconds if that makes sense. But warned them that if the line ever goes dead or becomes disconnected they wont know.
There is no POTS/Copper lines onsite.

Give them the provider number and let them work it out it has nothing to do with your equipment.

"...it did this before on the local copper lines too."

Sorry to repeat myself, but it is not a telephone-related issue, either copper, channel bank, or whatever (the "whatever" being you, for example.)

It is an alarm panel problem. Maybe the phone line voltage-check circuit in the panel is acting weird. Maybe you should put 48 volts DC at 25 mA (current limited from a good power supply) on the telephone line input and watch the voltage at the panel.