VoIP crypto with Diffie-Hellman? (Score 2, Interesting)

Just curious, but if we're talking about key exchanges over an insecure medium, why can't we do a Diffie-Hellman key exchange, similar to what is used for IPSec tunnel negotiation? It seems like VoIP devices could establish tunnels to remote endpoints via GRE and/or IPSec and pass their H.xxx protocol data over that tunnel. Is this not technically possible, or is it possible, just not scalable/cost effective?