Iphone speaker seems to act as half duplex when in facetime

This refers to my daughter's 4s iPhone.

When we are on Facetime, my 4s works as expected (full duplex). That is, I can hear what she is saying even if I am speaking at the same time. This, however, is not true for her phone. She cannot hear me through her speaker if she is talking at the same time. This is half-duplex, like speakerphones from years ago where only one person at a time could speak. Is there a work around for this, or should she just replace the phone (new, under warranty). I can't seem to find anything regarding this issue on these forums or FAQs. Any ideas?