I did not think on it but may be you can not use any phone like a industrial GSM module. If you will try to communicate you will found how to connect with only GSM module circuit. You would not need any other section of mobile circuit apart of module & RF section. I recommend you should buy a industrial GSM module SIM800L (low price) and enjoy.
Separating the module from the mobile phone & applying power to the module needs very critical attention. Instead of doing all these better go for GSM module.
As far as I know, all mobile phones support "AT" commands. There is no need to separate it from the rest of the phone, even if that was posssbile, just conect it via a serial link, either USB or using a conventional UART, whichever it supports.