How to forward from a SIP number via a SIM card to another number?

Question

SIP

Please tell me how to make such a bunch:
1. A call to a direct city number in zadarma.com
2. Forwarded to a piece of iron (with an inserted SIM card) using an internal free number
3. A piece of iron transfers the call to a given number via GSM ( through an internal SIM card)
This is necessary in order for the call forwarding to be free. For free, there is call forwarding to other phones, but it is paid. In this scheme, we receive an internal SIP call for free, and transfer to an external line at the rates of a SIM card (in our case, also free of charge).
I searched in Google, but I did not find an understandable solution for myself, most likely due to an incorrectly composed request.

Answer 1

The question, as I understand it, is abstract and in fact there are no "pieces of iron" now?
As a primitive option, used in our office as a "reserve" line (the CIP provider does not allow calls to 8800 numbers): Asterisk + unlocked huawei e1550. It works stably, you can cram a few of these.

Answer 2

If at a minimum of work and hardware, then use the SIPGSMGateway software from Octell. Put on "almost any" computer with Windows, stick whistles (the list of supported models is googled). Set up and use. The process will hang in the background quietly and calmly.
Of the advantages of this particular software, support for a significantly larger number of modems than modules for Asterisk (for example, ZTE).