loud buzzing sound on intercom calls

[Stewart]

I have a new setup with T26P phones running 6.70.0.140. On about 25% of intercom calls (apparently at random), when the caller is also using speakerphone, e.g. pressing intercom button when on-hook, a continuous loud buzzing is heard. Sometimes the caller hears the noise, sometimes the called party. If either party lifts the handset, or if the party getting the buzz speaks loudly, the buzz stops and normal conversation can take place. The problem does not occur if the caller is using the handset, or on non-intercom calls. The buzz occurs after the normal intercom warning beep is played.

Codec is G.722; Asterisk is set to re-invite the audio and I've confirmed that audio flows directly between the phones (on the same LAN).

Has anyone else seen this problem?

Thanks,

Stewart