Actually, it happens all the time, at many levels. But, what do you mean by dialog, exactly?
I mean, are they supposed to be talking about the weather

? Seriously, they get together with issues or problems that concern the general welfare of the faith but they don’t often see the need to discuss church practices (like the liturgy, which does not require manipulation) nor discuss theological introductions/innovations (the faith is solid).
I remember that they all got together when the most recent Pope of Alexandria was installed, but that was more of a social gathering. Then they got together to all agree on the status of the Patriarch of Jerusalem. There are regular meetings of committees of delegates for other purposes, such as the
Chambesy meetings and the
Orthodox-non-Chalcedonian dialogs.
I posted in another place that the mother churches assembled in 2009 to erect a new umbrella organization for the North American Orthodox, it took effect almost immediately and retired the earlier SCOBA organization.
And for smaller matters there is always the telephone …