def METHOD_NAME(msisdn: str) -> bytes:
    # Mimic how the MSISDN is encoded in ULA : 3GPP TS 29.329-f10
    # For odd length MSISDN pad it with an extra 'F'/'1111'
    if len(msisdn) % 2 != 0:
        msisdn = msisdn + "F"
    result = []
    # Treat each 2 characters as a byte and flip the order
    for i in range(len(msisdn) // 2):
        first = int(msisdn[2 * i])
        second = int(msisdn[2 * i + 1], 16)
        flipped = first + (second << 4)
        result.append(flipped)
    return bytes(result)