def set_phone_number_for_subscription(log, ad, subid, phone_num):
    """Set phone number for subscription

    Args:
        log: log object.
        ad: Android device object.
        subid: subscription id.
        phone_num: phone number string.

    Returns:
        True if success.
    """
    try:
        ad.cfg['subscription'][subid]['phone_num'] = phone_num
    except Exception:
        return False
    return True