I recommend you to read :

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.300.417

https://security.stackexchange.com/questions/2185/suitable-cipher-for-sms-encryption

As well as, there are android application called Crypto SMS .

https://play.google.com/store/apps/details?id=faris.com.cryptomessage&hl=en

The signal app already works as an encrypted replacement for SMS: https://signal.org/

thank you @Vijay Tiwari sir for making the time for answering my question.

but i haven't got my proper answer. may be I didn't framed it properly or something.

I just want to encrypt sms on my phone(any phone without internet) and then i want it to send to a number (specified by an operator). so when the sms is received on the operator server then operator can decrypt it and read it.

so is this possible to do somehow? and i don't want the internet in between for any purpose while performing all these.

The short answer to your question is probably ”no.” If what you are asking is if the SMS feature of your feature phone can be made to encrypt and decrypt messages, device to device, without user intervention, it is hardly likely. The feature is not open to user modification. Moreover, both devices have to be altered in a similar way, but the application programming interface (API) of heterogeneous phones may not be the same.

iOS devices do encrypt messages device to device by default. However, they use the same feature for both this function and SMS. SMS messages to non-iOS devices are not encrypted. Of course, these are ”smart” phones, not simply ”feature” phones.

There are encrypting messaging apps for Android smart phones.

Device to device encryption should not be relied upon for life and death applications. Person to person encryption should be used.