The other way of doing it is to setup an email account with your mobile phone company (if it's a UK phone), which can send you copies of text emails it receives. The email then gets forwarded to you by your network.

You can then sign up for email alerts through the EasyUpdate menu on united.com, which will work for every flight. I know that Orange can accomodate this system, but am not sure about O2, Vodafone, Virgin Mobile or T-Mobile...