I've been having the same problem listed here, but when I attempted to reconfigure things, when I sign into the old chat, I get the message
"Google Talk will be replaced with Hangouts on June 26, 2017"
and when I click on the handset, the dialer pops up, and says
"To make or receive phone calls in this browser, please enable Hangouts"
Now, oddly, the destination Google Chat still shows up in phones, and I have it checked and all that (have also tried to uncheck and recheck it, also tried deleting it and setting it up again) but in my case, nothing works. I get 503 errors on outbound calls (though I can reach the echo server), and inbound calls ring four to six times then go to voicemail (Google Voice voicemail -- I can see the call in the Google Voice logs).
Anyhow I thought I'd post as the previous post (from a month ago) said they were able to load the dialer from within the old Chat, which seems to no longer work.