Maybe you can transmit to an activated phone using Bluetooth and either tether or remote operate. The BT hardware should not be too hard to find. Writing code for it to connect to an app in your phone is a different kind of hurdle. One would need to develop or locate sample code for both the app and the firmware. Negotiating a cable modem connection to a flip phone might be easier, I think you may be able to use a serial Modem mode.
Can we assume the SIM card was already activated and worked in another phone otherwise?
I think the Adafruit fona module is the best bet if you want to stay on the same track at $44
Not a lot of cellular modem dev kits but here are a couple:http://hackaday.com/2015/04/05/a-cellul ... data-plan/ https://www.kickstarter.com/projects/sp ... imple-datahttps://www.particle.io/products/hardwa ... ar-dev-kit
Not cheap stuff:https://www.semiconductorstore.com/Cell ... Kits/C/429
"LTE Dev Kit" found more stuffhttps://www.arrow.com/en/products/1963/ ... YgodNy8Khwhttp://www.mouser.com/new/adafruit/adaf ... ona-board/
Unrelated but Cool new product I found while searching:https://www.digikey.com/product-detail/ ... fgoda3AFAQ