Her testimony, truthful or not, is not hearsay. She was a primary witness, albeit subject to the limits of telephony and the length of the call.
Hearsay doesn't refer to the human ear. What the ear hears is admissible, if the ear was listening to an actual event. Hearsay is second-hand accounts. So and so told me such and such is not admissable, whether by ear or by email, letter or whatever.
To the extent the prosecution uses her to testify to a declaration that Trayvon made in order to prove the matter asserted in the declaration that would be hearsay. Such as "Trayvon told me on the phone it was raining" in order to prove to the jury that it was raining.
An example of such a statement I expect the prosecution to introduce would be her testimony that Trayvon told her that he was being followed, in order to establish the fact that Trayvon was following her.
Nevertheless, even though some of these statements may be literally hearsay, I expect the court to allow all or nearly all of these statements into evidence under the residual exception if necessary.