xfinifty voice analog or digital

xifnity voice modem have tele port connect to Rj11 port.    RJ11 is analog . why xifinity voice connect with analog phone line?  The other end also connected with Rj11 on analog rotary phone.  Voip should use Ethernet connection.  Xfinity voice is voIP service.  voip use voip/sip handset connect with Ethernet. 

How to connect xfnity voice to voip phone handset with Ethernet cable ?