Welcome to the Xfinity Forums. I can help with your Voice2 go issues. Can you please confirm rather or not these are the steps you're taking to add the phone numbers?
How to Create Personal Phone Numbers for Voice2Go
Voice2Go is an Xfinity Voice feature that lets you make and receive calls with your home phone number through the Xfinity Connect app. You can also create up to four additional Voice2Go numbers for use with any WiFi-enabled device through the Xfinity Connect app. With the additional numbers, you can make outgoing calls, send text messages and forward incoming home phone calls to your mobile device.
Each Voice2Go number must be assigned to a separate user on your account, and each user may only be assigned a single Voice2Go number.
Learn more about Voice2Go. The Xfinity Connect app is available in the App Store (iOS) and Google Play Store.
Create a Personal Telephone Number for Use with Voice2Go
Note: To add a personal phone number through Voice2Go, you must first create a secondary user to assign the number to. Learn more about adding additional Xfinity usernames to your account.