We’ve known for weeks that Amazon has been working on ways to give Alexa-powered, voice-controlled devices the ability to make and receive phone calls, but a newly published patent details how it could be done.

The patent also shows that the idea has been in the works for more than four years.

The system described in the patent would link an existing cellphone number with the Alexa device, and includes a provision for identifying authorized users by their voices. Alexa could let the user know a call is coming in, and then pick it up and route the call over its speaker when the user gives the go-ahead. Users could also initiate calls via Alexa.

Call charges would be billed through the user’s cell carrier, but once the call is routed through the Alexa device, the cellphone would be left out of the picture. Instead, the Alexa device would send the call to the carrier through a type of connection known as voice over Internet Protocol, or VOIP.

The best-known VOIP carrier is Microsoft’s Skype service, and Amazon is said to be anxious to get in on similar phone applications.

The company is reportedly gearing up to announce Alexa-enabled phone service sometime in the next few months. In March, Recode quoted unnamed sources as saying that new hardware optimized for phone calls and in-home intercom applications is undergoing beta testing.

Amazon hasn’t said anything publicly about its plans for phone service, and it makes a habit of refraining from comment about its patents. But the patent suggests that most of the heavy lifting would be done in the cloud and by cell carriers rather than on the Alexa device.

Alexa is already playing a role in intercom applications through a startup called Nucleus, which won $5.6 million last September in an investment round led by Amazon’s Alexa Fund. Nucleus’ smart-intercom tablets incorporate the Alexa AI voice assistant.