DD1 loves talking on the phone, she's not quite 2 yet but could talk under water with a mouth full of cement (much like myself), she has actually called DP while 'playing' with the phone before.
She tries to say hello and at the end she says bye or c-ya and hands me the phone.

I will be teaching her emergency numbers once she's a bit more controlled (right now it's chatting to ppl for fun nothing else) and how to contact us.

My younger brother used to answer the phone from about 3yrs of age, he had been taught to say "hello Ji speaking" it was so damn cute.

I know other children though who have no phone manners at all, their like 12 and still can't string together a hello or a proper goodbye, those children though were never 'allowed' to answer the phone, it was drummed into them the phone was not to be touched and they were not to answer it, mum or dad were.

I think it may just be she's a nervous person, maybe do some test calls from your mobile to home, some 'practice' calls on a fake mobile or something, get a friend to call her at home and see how she goes.