Okay yeah, as much as I like Steins;Gate... the phone is total bulltrout. It's the most inconvenient way of making choices I've ever seen in a visual novel. Whenever you can respond to a text message you get, you can sort of choose what 'subject' you want to bring up, but the problem is that you don't see the message you're going to type until you can already no longer back out of sending it. Which is stupid and makes getting the 'correct' choices incredibly annoying unless you follow a guide step-by-step, or painstakingly write down what to send.
I played through the game twice then just used a guide to get to the true ending. Looking back on it, I might have doubled my playtime had I tried to find it myself.