Are there any hearing aids that allow you to take calls from an Apple Watch directly through the Apple Watch's cellular network connection?

Starkey hearing aids (Halo hearing aids) allow for this. It is actually pretty neat all that you can be connected to with these small devices that not only help you hear better but connect you to more in your world/environment. 

Unsure about the Apple Watch cellular network. I have fit both Signia and Phonak instruments that stream Bluetooth. The Phonak Audeo B Directs allow you to answer calls by pressing the button on your hearing aid without having to even touch your phone. Currently, these features work best on Phonak's MFA (made for all) platform that integrates with both Android and iPhone. Signia's Nx and Charge&Go hearing aids work well with iPhone, but not yet with Android.

I believe Starkey is the only manufacturer that allows for streaming audio from the watch (with limitations). The other manufacturers that are "made for iPhone" only allow for use of their App (based on manufacturer), which would allow for adjustments to the hearing aids such as volume and program selection - but not streaming audio. I would expect this will change in time.