Polycom (formerly Obihai) OBi202 2-Line Voice-over-IP Telephone Adapter for $60 with coupon 93XRK97?. Normally sells for about $80.
This two-line version can handle incoming and outgoing calls for two separate phones and numbers. It is meant for a small business that may want to receive calls on one phone and make calls on another.
Google Voice gives a free phone numbers, free home phone service, unlimited calling within the US and Canada, text messaging, and voicemail with transcribe.
As an alternative to using this box with a cordless phone, you download the Google Voice app on any Apple or Android smart phone. The app can do incoming and outgoing phone calls over wifi, without paying for mobile service.