create a short code for that user of....
Code: *
Feature: Call Pickup Extn
Telephone#: 1234(Extension you want to Pickup)
I am just using * as an example, so in this case if you heard extension 1234 ringing you could press talk on your analog phone and then press * and you would be talking to the person that was ringing 1234.
I personally wouldnt use star, if you use star, you wouldnt be able to dial any other short codes from that phone, so you could may be use #.