If the phone is the only option, then I would recommend calling her. I assume if you two always flirt anyway then she is expecting it or wants this to happen anyway. A call is much more personal and shows that you care, just start off with a casual conversation that you'd usually have (including the flirty stuff) then ask her out. :)
Edit: Alright, if you absolutely have to do this over text then do the exact same. Startup a conversation, just a simple, "Hello." or whatever else, text back and forth normally, and then just flat out ask her. Girls usually appreciate a guy who will just take control.
As a girl I actually asked my current boyfriend out over the phone because I was too nervous in person and his friends were glued to him, so I understand your situation.