unique keys / foreign keys on two tables

I'm designing the schema to store a config from our switchboards.

As with all PBX's the key is the dialed number which is either an extension
number or a group (hunt/ring/pickup) number.

I have two tables, one for extensions and one for groups, basically

ext_id int4 primary key
ext_desc text
....
....
....

and

grp_id int4 primary key
grp_desc text
.....
.....
.....

I now need to be able to ensure the id field is unique across both tables.
Presumably I can do this with a function and a constraint for each table.
Does anyone have examples of this?

Next I have other tables that refer to *destinations* which will be an ID that
could be either an extension or a group. Examples are 'Direct Dial In'
numbers which could point to either. How would I do that?

--
Gary Stainburn
Group I.T. Manager
Ringways Garages
http://www.ringways.co.uk