They run on Amazon EC2 with instances that have Asterisk; Digium is one provider of hardware for these situations but I don't know how that's incorporated into their architecture
A better alternative to Asterisk: FreeSWITCH - www.freeswitch.org
We're slowly migrating most of our PBX/IVR apps from Asterisk to FreeSWITCH. I agree FreeSWITCH is higher quality code, but unfortunately there is a much smaller developer/support/hardware ecosystem with FreeSWITCH right now.
"Better" is very relative, as the sibling poster rightly points out.
They use some Asterisk. There is also a significant (and well-known) commercial component proprietary to the enhanced voice application services industry, although there are various reasons for which I cannot disclose what it is.