Elves would surely make use of arboreal communications (
not so fanciful!) with obvious the capability to make 'trunk calls'...
Humans, having provably adopted early iterations of
RFC 1149, here on roundworld, should certainly have some form of primeval
IPoAC established on dorfworld.
The bearded buggers with their love of levers
might have a
Semaphore system overground (with enough hilltop sites built up) or just a spooky-action-at-a-distance
lever2indicator thing if they're able to initially trek the distance with the components needed.
Goblins? Well, they've probably got communication daemons of some kind.
With an established diplomatic relationship, inter-species communications could be via 'loan' terminii to one kind of communications method (or bothmkinds, according to direction of traffic) or by bi/multilaterally-founded settlements which use a
sneakernet interface between the two or more systems provided by the home civ. If things run smooth. (The Dorfish link might easily suffer from Sock Error, etc.)
If all that is too
easy, then of course a tree-link could be rooted (up!), crossbow bolts can pierce the sky in huge quantities, vile forces of darkness can obscure lines of sight (while distance itself might make quantum leverage disentangle, or just let loose the gremlins) and temples might be built that disrupt the dispersal of diabolic dispatches.