Recieved a txt from girlfriend but came from different no. Is it some type of spy app?

[Android Central Question]

Hello. Would be greateful for any feedback please.

My girlfriend was adamant that she txt me but I never recieved a txt from her. A few days later, I recieved a txt from an uknown no. It was clearly a message my GF would txt. I called her and asked and she confirmed that she txt but when I asked her why she had txt off someone elses no and whose was it she was confused. She had sent the txt off her normal no. I got off the phone to her, called the one that her txt had come through on and although it rang, there was no answer. I rang her back and asked if shed had any calls from me or a txt and she had not. She then txt me again and it came through like normal on her no. Neither of us recognize this no but the initial txt message is still logged in my phone as being from this unkown no. We attempted to call and txt the no multiple times and it just kept ringing and then going to voicemail - an 02 no. Then we thought it was strange as I have not recieved at all that other msg my GF sent me. Any clues as to how or why this would happen? Thanks for any feedback x