Hello @Eunodio Gomez
The command to configure the OnPremLineURI is:
Set-CsUser -Identity username@keyman .com -OnPremLineURI "tel:+xxxxxxxxxxx"
If you are receiving an error trying this way, please share the error with us.
Please also share the MicrosoftTeams Module version you are using.
To find the version, please run Get-Module on your PowerShell and find the MicrosoftTeams line on the list.