One thing to be mindful of is QoS with GRE tunnels. QoS will be kicked off when the tx-ring of an interface is full and packets will be dropped when the interface is congested.
These two attributes aren't directly correlated with a GRE tunnel, so it's best to do a hierarchical shaper. This will limit the GRE to a bandwidth value and won't oversubscribe the WAN.
Example:
class-map voice
match protocol rtp audio
class-map skinny
match protocol skinny
policy-map shaper-parent
class class-default
shape average 1544000
service-policy shaper-child
policy-map shaper-child
class voice
priority 1000
class skinny
bandwidth 75
interface tunnel 1
service-policy out shaper-parent
For your one way voice problems you need to make sure that each of your sites have unique addressing space. If they have overlapping address space you're going to run into problems when those phones want to talk to each other.
Otherwise, every site is going to need reachability from phone-to-phone at each site. If you pull up the phone webpage during a call with one-way audio you'll see the address it's trying to connect to in the stream statistics.
hth,
nick