Not sure if this really is what you are looking for, but if you put *80 in front of the phone extension you are dialing, and the phone supports it, the remote phone will go off hook and they can answer handsfree, if they are not on the phone. handsfree intercom.
Example: extension 100
Dial *80100
On extension you hear the alert tone and you can immediately talk to whoever intercomed you.
This is not exactly PTT, but similar.
Additionally, you can page through the phones by setting up a paging group.
When I am working remotely, I often use this to find a free technician rather than calling 3 extensions one at a time, or going through customer service queue.