Accommodation of devices not purchased from or originally provisioned for service by 8x8 is on a commercially reasonable effort basis. These devices are not guaranteed to be supported on 8x8 service.
Do you have Yealink phones that you would like to reuse with 8x8 service? That’s great! You can reuse existing Yealink phones with 8x8 as long as they are a supported model with a valid factory installed device certificate. It's crucial that Yealink phone certificates are factory installed and are not added later.