It doesn't quite work like that.
We do not do your work for you.
If you want someone to write your code, you have to pay - I suggest you go to Freelancer.com and ask there.
But be aware: you get what you pay for. Pay peanuts, get monkeys.
Or go to MSDN: https://msdn.microsoft.com/en-us/library/system.io.ports.serialport(v=vs.110).aspx
] and the manual for the PBX you are using and start writing it yourself. You will need both in order to work out what to send and receive in order to understand the communication data.