Martha's mother Francine did not speak to Harold Saxon personally. In an answerphone message for Martha she said that the information came from Harold Saxon, but the message was delivered to her by an unnamed male character at Lazarus' experiment. The 'mystery man' told her:
"He's dangerous. There are things you should know."
He then whispers further details to her, but we don't hear what they are. Francine finds the Doctor and says that there is "nothing but death and destruction" around him, so evidently this is part of what she was told.
Later, Francine leaves a message on Martha's answerphone saying:
"Martha, it's your mother. Please phone me back. I'm begging you. I know who this Doctor really is. I know he's dangerous. You're going to get yourself killed. Please, trust me. This information comes from Harold Saxon himself. You're not safe!"
In previous stories (for example, the TV Movie) The Master has claimed that he is the benevolent Timelord and The Doctor is the evil one - effectively reversing their roles. It is possible that Francine was told something along these lines.