Line URI must contain a normalized, unique primary phone number when a user is enabled for dial-in conferencing, Enterprise ...

Line URI must contain a normalized, unique primary phone number when a user is enabled for dial-in conferencing, Enterprise Voice, PC-to-PC calls, or remote call control. Use the E.164 format and include the prefix "tel:" (for example, tel:+14255551297). If there is an extension number, add it to the end of the line URI (for example: tel:+14255551297;ext=51297).