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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.

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  • \$\begingroup\$ It don't work that way, fella. This is so wrong, I don't know where to start. \$\endgroup\$ – JRE Nov 5 '18 at 20:02
  • \$\begingroup\$ you are asking how to hardwire a landline phone to a cell phone ..... why would you want to do that? \$\endgroup\$ – jsotola Nov 5 '18 at 20:04
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    \$\begingroup\$ you are under the misguided impression that you only require a SIM card to connect to a cell phone provider and that the rest of the cell phone is somehow just a decoration \$\endgroup\$ – jsotola Nov 5 '18 at 20:07
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    \$\begingroup\$ You can do something like that with an pc acting as an ipbx (using, e.g. a software like asterisk), to which you will plug both some gsm gateway hardware and a pstn gateway with a fxs port. Also, you could make some cool stuff with this setup, like custom dials. This is however entirely off-topic here, since it is hardware recommendation and also not electronic design. \$\endgroup\$ – dim Nov 5 '18 at 20:57
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    \$\begingroup\$ Buy a Home PBX which has a cellphone uplink. They cost about €200. You need a SIM card with internet access enabled (and an affordable payment plan) and an IP phone account. \$\endgroup\$ – Janka Nov 5 '18 at 21:49
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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.

Internal view of digital electronics in old rotary phone

(Image source - Sparkfun Port-O-Rotary tutorial)

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.

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  • \$\begingroup\$ Sam Gibson. No need for that stuff. There are home PBX out in the wild with a cellphone uplink. They are about €200. +1 anyway for curiosity. \$\endgroup\$ – Janka Nov 5 '18 at 21:48
  • \$\begingroup\$ @Janka - Thanks :-) Yes, there are other ways, but just buying one of those PBX takes all the fun out of it, doesn't it? ;-) Also, if this wasn't going to be an electronics design question, then it's more like an off-topic shopping question. Therefore I decided to treat it that the OP was asking about a design question :-) +1 to your comment giving the OP the PBX option. \$\endgroup\$ – SamGibson Nov 5 '18 at 21:59
  • \$\begingroup\$ Thank you so much! my questions are 1) price of this modifications, 2) how hard is it to build and 3) what components do i need? \$\endgroup\$ – The Shwarma Nov 6 '18 at 0:06
  • \$\begingroup\$ @TheShwarma - If you have to ask how hard it is, even after reading that tutorial, then it's probably too hard for you :-( This site is not a good fit for "whole project" questions. As explained in the tour and help center, questions here must be reasonably scoped, not require a book to answer them, and need to have been researched first. Within that scope, I have pointed you towards one possible approach for you to research. However there are several options you could choose, which will affect your questions (1) and (3). When you are doing your design, feel free to come back with questions! HTH \$\endgroup\$ – SamGibson Nov 6 '18 at 16:22

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