Based on my past experience of using Sipura/Linksys ATA for a number of years, a good switching transformer would not give too much AC hum when ordinary corded phones are used.
You may have the previous discussion of AC hum in the following link:
http://www.obitalk.com/forum/index.php?topic=367.0When I visited a very big electronics fair in Hong Kong recently, I have already approached different transformer suppliers and got a reply that ordineary switching power adaptors are not good enough for audio device (such as OBi110 and OBi100). Most of US/CAD, European customers request the swtiching transformers to pass certain performance standards which are often required for IP Phone or other audio products.
The unit price is around 2.6USD.
A very good piece of device OBi110 or OBi100 which comes with a general grade of switching power adaptor will DEGRADE the audio performance when OBi devices are comapared with the Sipura/Linksys ATAs.
Most of customers does not mind to pay several dollars more if the AC hum is removed.
YH