1. Business
PABX is an automatic telephone switching system, belongs to traditional communication industry, the companies care more about brands, not keyword, care more marketing management, not traffic. And such technology is out of date (my point, maybe I am wrong), people show little interest on it.
2. Traffic
Google Adwords Data
[pbx] high competition, 40500 global, 9900 local, $14.37 cpc
[pabx] high competition, 33100 global, 590 local, $11.88 cpc
3. Potential
This domain name maybe can be developed as an affiliate or directory site, but it's hard to find company will pay you commission.
4. Extensions
all major extensions have been taken. But none of them is developed.
$80 to end user