The Obi 100 & the Obi 110 are small electronic boxes which allow you to connect your standard telephone (an analog device) to the internet (a digital service) to make and receive VoIP calls. These boxes are called ATA’s, Analog Telephone Adapters
The Obi 100 allows you to make and receive calls over the internet only – VoIP calls only.
The Obi 110 allows you to make and receive calls over the internet (VoIP) or over your existing telephone line. (your Bell Canada telephone line, for example)
Let me explain the difference by the use of a typical example for someone who wants to ‘try’ VoIP.
Right now, you have a residential telephone line from Bell Canada – I’ll refer to this as your ‘regular line’. You also have a HIGH SPEED internet connection from Bell, Rogers, Shaw, Videotron etc.
You want to ‘try’ this VoIP stuff but you do not want to cancel your existing telephone line yet.
Using the Obi 100:
You connect your Obi 100 to your HIGH SPEED router.
To make & receive calls, using your ‘regular line’, you must have a phone plugged into a wall jack.
To make & receive calls, using VoIP, you must have a phone plugged into the Obi 100.
This would mean that you either need 2 phones or you must switch a phone back and forth between the wall jack and the Obi 100.
Using the Obi 110:
You connect your Obi 110 to your HIGH SPEED router and to your existing telephone jack.
You can then use the same phone to make and receive calls from BOTH your ‘regular line’ and from VoIP.
Yes, the setup the Obi 110 adds one more step over setting up an Obi 100 but it is no big deal.
All Obi ATA’s come with complete, simple set up instructions written for ‘non-technical’ people like you and me.
Note: If you buy an Obi 110 and do not connect it to your telephone line, it simply works like an Obi 100 as described above.
ATA definition: http://en.wikipedia.org/wiki/Analog_telephone_adapter - a bit technical, so just settle for the first line of this post. 
