There is no rule you must or must not put "+" or area code. The format of forward number should be exactly the same as what you input when you manually send sms.

Please check step by step as below.

• Check the format of destination number. The format should be exactly the same as what you input when you manually send sms.
• Check the sms encoding your operator support. The default sms encoding is UCS2. If your operator does not support this encoding, set sms encoding to 7bit. (Some operators in Europe, India and American only support 7bit encoding.)