The GF describing what Martin said to her on the phone, or what she heard on the phone, is not hearsay and I defy you to prove, with authoritative cites, that it is.
She may testify to both what Martin said, as well as what other voices picked up by the telephone's mic. She may legally say, this was Martin's voice. This was not Martin's voice. She can also describe the change in tone she heard. She may not speculate as to the cause of the change in tone.
Have a nice day.