If you are really interested in this service, it is better to contact the detective Agency. It will cost a lot (about 500.(e.), but the likelihood that you do get the ability to read SMS correspondence of another person, in this case large. If you have the financial ability to pay for the services of a private detective, then go for it!
You can also install on the phone of a spy software. With its help it is possible not only to read other people's SMS, but also to listen to telephone calls. Note however that for this you need access to the phone of the person, because the program will first have to install on his phone.
In conclusion, we note that whatever the reasons for your desire to know that someone writes another person, remember that it is illegal. Everyone has the right to privacy, and check other people's phone calls and SMS are entitled to only law enforcement agencies, and that in case of real threat of committing a crime or conducting investigations.