Hi,

I don't see any issues on routing voice traffic over GRE tunnels aside from any issues on running voice over any IP network.

You need QoS to prioritize the voice over other bandwidth-consuming traffic, and give it a special treatment since voice cannot tolerate network-related delays or jitter.

Also, GRE being a tunneling protocol, adds overhead to the voice packets, so keep that in mind when setting the MTU.

To secure the voice, you can use IPsec/GRE to encrypt and protect the traffic, but it has the drawback that it allows more overhead.

A lot of things depends on what you need.

Federico.