But how to add it in elastix...like as IP phone ?...I suspect that you'll find that used analog telephone adapters (ATAs) + standard phones are less expensive than IP phones. Is there any reason why that wouldn't work for you?
To find inexpensive ATAs go to eBay and search for obi100 and obi110 adapters. They're obsolete for connecting directly to Google Voice, but still work great as sip devices. You should be able to find them in the $15 to $20 range, delivered price with power adapter.