Scott,
Easiest way would probably be to create a segmentation. Javascript works as well as Josh mentioned.
To do it using segmentation, using the info above as an example, you'd:
- Create a segmentation called "var1"
- Make a "s" and "t" segment
- On the landing page, segment the element you want to be dynamic with the segmentation you just created.
- Change the number for the "t" value to (1.888.666.6666) and "s" to (1.888.555.5555)