Okay I have an idea. It may be silly, but who cares, it might save you from being pranked and might give you a funny story to look back on. Okay, you said you served them right? So you know what they sound like I assume correct? If so, meet up with a friend. Give them the girls number and have your friend call the girl on speaker with their phone. If they don't want there number to be revealed use #67 to privatize their outgoing number. When they answer and ask who you are have your friend pretend to be some salesman or something, and offer some fake ass sale. Example: A free house gutter cleaning service or something. You are only doing this just to get a good hearing sample of their voice to make sure that it's actually them and not some random ass person. You are having the friend talk so they don't know it's you who is doing this. It doesn't matter if they are interested or not. Once again, you are only doing this just to make sure the number is actually theirs and that they didn't give you some random ass dude's number. Hope this helps, good luck. If you happen to do this, let me know the outcome.