Once you set up an account, just follow the steps listed earlier. Visit the other answers provided in this Q&A for steps on how to install a spy phone app on your old i Phone, and then let her have it.However, bear in mind that cellphone spying can get you in trouble with the law, depending on where you live. The catch is, OS updates will wipe your spy phone install. You could give her another phone, that you installed the spy app on, and then wipe and restore her phone in front of you, so that she wouldn't think you were spying on her - but that's the sort of thing that, when you do it, might encourage her not to cheat.
In short, I am trying to say you will probably not find a one-off solution to account for all of your communication needs. You can get numbers from a tonne of countries (incl. Numbers are really cheap, and you can pay per-minute or buy an even cheaper minute package.
I suspect that my girlfriend is still cheating on me after I found out when I borrowed her phone and checked her Facebook, Skype and Viber, we had an argument, now she's saying that she is not cheating anymore but I can't believe it because she won't let me check her phone ever and she's using a code I can never check it even if I get the chance to.
I just want to find out whether I still can trust her or not. If she was already cheating, and now she won't let you check her messages, then she's probably still doing things that will hurt you.
She knows this, so she of course will not let you check her messages.
The thing is, was she actually cheating on you before, or did you assume she was cheating based on her messages?
If you want something with even more features, try out Viber. A direct competitor to Skype, offering free user-to-user calls and (mostly) free calls within the U. While you could use Hangouts as just another messaging app, that’s the last thing we need.