i recently got in a Goodwill store one of the 2500 model phones https://en.wikipedia.org/wiki/Model_500_telephone#Model_2500 and i want to use it. the problem is that i don't have a phone line in my house, just a cell phone's SIM card. is that possible to build something (like a box) that in one side of the box you put SIM card, other side a power source and in other side a RJ connector https://en.wikipedia.org/wiki/Registered_jack? i need this box to emulate the SIM card as a RJ connector and use the phone.
You said in a comment:
can you find any solution or a way?
So now that you have removed the requirement in the original question, for connecting to that POTS phone via the RJ-11 connection - yes, there is a way.
Almost the same project was done as a Sparkfun tutorial called the Port-O-Rotary in 2005.
The main difference in your case is that your phone has a keypad, and the POTS phone in that tutorial has a rotary dial.
It's too broad to go into every detail here, but some of the challenges in the project include:
Choose a GSM board which fits in your POTS phone case.
Consider which battery will also fit in the case and how you're going to charge it.
Choose an MCU platform, which you will program to control the GSM board, interface to the keypad etc.
Decide if you are going to use the existing on-hook/off-hook switch, or design another method of ending each call.
Test whether you need to design additional analog electronics, to interface between the existing POTS phone mouthpiece and earpiece, and the GSM board analog input and output.
Decide whether you'll use the existing "ringer" in the POTS phone (whatever that is) and design the necessary electronics to drive that, or replace it with a low-voltage buzzer etc.
There will be lots of reverse-engineering for you to do (e.g. keypad and handset connections etc.), as well as programming whichever MCU you choose to use to "drive" your modified phone.