How do you think other traffic gets by, on a daily basis?
Call one of them; if unsure, call the unit you are closest to. ATCOs of adjacent units do talk to each other and pass information by landline. Sometimes they are even co-located. If an ATCO thinks you would be better off passing your message to someone else, he will tell you.
Surely all glider pilots are taught this from an early age? It's pretty basic stuff.