Having deployed a number of the new 4331 routers to replace 2800 series routers as voice gateways, there's been some quirks...
The first being that unicast music on hold stopped working. Identical configuration to the 2800 routers, but PSTN callers just hear silence. Eventually found that setting the CallManager service parameter Duplex Streaming Enabled to true fixes this problem:
I also hit a nasty bug in that 4000 series IOS XE releases 3.15.0, 3.15.1 & 3.16.0 aren't fit for purpose as voice gateways, whenever the router reloads the dial-peer configuration (along with NBAR & some ACLs) disappears. Why these versions aren't deferred releases yet I don't know?!
UPDATE: Now to cap it all off the 4300 series is undergoing a recall.