Your work has brought you into contact with this lady, you should leave it at that. You could be charged with professional misconduct if you start a relationship with a client. Block her from your phone or your messaging service. She may see you as a good prospect but your professional relationship with her must take precedence.
You have already been in potential trouble at work for using your work phone to call your girlfriend so you are obviously not without female company - tell this lady that you are already spoken for, if she should go as far as proposing to you.