MIC and Loudspeaker common

I am currently trying to connect a phone to a door intercom system. Everything is done except for what I thought would be the simplest aspect.

The door system mic and loud speaker share common GND, where as the phone mic/loudspeaker do not.

I have tried joining the GND on the mic/loudspeaker on the phone but it does not work (phone goes a bit mad) I have also tried doing the same but with a diode across the MIC.

Any ideas?

Thanks

Don't worry, I sorted this.

delete as appropriate

Thanks