Risk of putting VOIP ATA in router FW "DMZ" mode

New Linksys PAP2-T VOIP ATA loses dial tone ( & no light on L-1)every night, and / or ~15 - 20 min after shut PC down. Reboot, after few min, get dial tone. Acct's w/ Future Nine. When it's working, works great.

Power is NEVER cut to my 2Wire 2701 HG-B gateway or the ATA.
Gateway's ethernet, DSL & internet lights are always on in morn (before boot PC) when the ATA has no dial tone.

Powering gateway off / on also fixes problem, then have a dial tone for (? hours) or until shut PC down.

AT&T's Lev II DSL Tech said putting *only* the Linksys ATA in DMZ Plus mode, should solve prob of losing dial tone when PC is off. Seems to work so far. The ATA only needs one port, per Future Nine. The gateway auto picked up ATA & lists as a "computer" on home network.

?? Does putting the ATA in DMZ Plus Mode pose additional security risks? This is from the FW DMZ setting page:

In gateway FW, the SIP application (I had to "user define") is assigned to my DESKTOP, NOT the ATA. Could that be the prob? Wondered w/ SIP assigned to PC, when PC's off, does SIP no longer work? Tech wasn't sure - said possibly.

The FW allows assigning SIP to ONE "computer" at a time, so could switch SIP to the PAP2.

Anyone had experience w/ these settings, and understand the risks of putting the ATA in DMZ mode? Read this in another post, but don't know how much risk since it's an ATA, not a computer:

Thanks.