"""Convert milliseconds since epoch to an ISO 8601 timestamp string in UTC timezone.

The function takes a timestamp in milliseconds since the Unix epoch (January 1, 1970) and converts it to an ISO 8601 formatted string. The output string ensures the timezone is represented as '+00:00' for UTC, even if the internal conversion initially appends 'Z'.

Args:
    ms (int): The number of milliseconds since the Unix epoch.

Returns:
    str: An ISO 8601 formatted timestamp string in UTC timezone, with the timezone represented as '+00:00'.
"""