It seems like the avatar parameter comes in webhooks, where is the link to download the image from the viber server.
If the photo download is available from the client's server, it will take 1 hour to finalize it.
But it is better to check the availability on the example of a specific OneBox, because if some additional manipulations are required, the score will increase.
If we are talking about I2CRM, as far as I know, they do not transmit contact image data.
If we are talking about a messenger - there in the documentation for the site
https://developers.facebook.com/docs/graph-api/reference/user it is also indicated that a photo can be requested in the profile_pic field, but it is not clear there whether the photo of the requested one will be returned contact.
We are also talking about a separate api node
https://developers.facebook.com/docs/graph-api/reference/user/picture/ , and a token is needed. But it is not clear whether it is possible to get a photo of the user who writes to you in this way.
In general, the same tests are needed here on a specific OneBox and a fresh message (because they write that the URL in the photo will be temporary) - such tests will take 1 hour. After that, if any of the options turns out to be working, it will take 2 hours to finalize it in push notifications.