Those devices combined use very little power. If that's all you plug into a UPS, you should be able to accomplish what you want with a 700-1000VA model. When shopping, look at the UPS specs, and confirm that it has at least a 12V 7AH (Amp-Hour) battery, like these:
http://www.zbattery.com/b-b-12v-7ah-sealed-lead-acid-battery?sc=2&category=60816Typically, the next step up model UPSs, around 1500VA, use two of these batteries. The next step down (500VA) may use a smaller battery -- not recommended for your run time requirement.
It used to be more critical to use a UPS with high-end features like pure sine wave inverter power, but for simple devices like yours, the lower-priced UPSs with stepped/approximated sine wave are ok. The only device you might have some trouble with is the DECT phone, since they usually have an old fashioned transformer AC adapter, vs. switching power supply adapter. You might get some hum when on battery backup, but the obvious solution to this is to keep an inexpensive corded phone for emergency use.
I've used lots of UPSs over the years, and I used to sell them to data center customers. In this size range, APC generally has the best quality and reliability. I use their top-o-de-line SmartUPS products, but their cheaper models would be fine for this purpose.
Just keep in mind that none of this will help if your 30-minute power outage also impacts the U-Verse equipment outside
However, it's a good strategy otherwise. I have FiOS service, and my FiOS ONT (Optical Network Terminal) in the garage came with its own dedicated UPS for this very reason (actually made by APC for Motorola/Verizon). That, plus my indoor UPS connected as you describe, can keep my telco service up and running for an hour or more.
Finally, remember that you'll need to replace the battery every 4-5 yrs, depending on ambient temperature, number of cycles, etc.
