I would like to turn my old iPad one (with 3G) into a phone. All you need to do install an app called [Ac!d]TelePad from the author Bassem Kassem (of course a SIM card allowing for calls is required).
The only “problem” is, that this is a Cydia App, not an App-Store App. Cydia is an alternative Appstore and you have to jailbreak your iDevice in order to use it.
Searching for iPad 1, iOS 5.1.1 (which is the latest supported version for iPad 1) and Jailbreak will ultimately lead you to GreenPois0n Absinthe 2.0.4, which seems to do a great job, but did not run on my Yosemite mac. The app opens but just disappears without any feedback.
Searching for another little while brings up redsn0w_mac_0.9.15b3 (e.g. from here). According to its readme file it is a
a lightweight, multi-platform jailbreaking, unlocking and customizing tool for the iPhone 2G, iPhone 3G, iPhone 3GS, iPhone4, iPhone4S, iPod touch 1G, iPod touch 2G, iPod touch 3G, iPod touch 4G, iPad1, and iPad2
I started the app (after making a backup of the iPad and resetting it). It worked just fine. Afterwards I could install all previous settings from the backup. The iPad worked as before. Just one difference: Cydia is now installed on my iPad.
In Cydia I added the resource http://BassamKassem.myrepospace.com. Doing this one can simply search for the app [Ac!d]TelePad, install it and then you can use your old iPad as phone 😀