In most countries, a dash ‘-‘ in a phone number exists to separate numbers into smaller groups, or even different exchanges.
When telephone networks first emerged with manual operators (people who sat at a switchboard directing call traffic), there was a need to have a smaller series of local exchanges.
These exchanges often acted like smaller versions of the area codes of today – each town or area would have its number exchange.
The change between the exchange and the local subscriber number were often denoted with the dash ‘-.’
These days, telephone networks are heavily automated. However, the individual exchanges still exist and help direct telephone traffic to the right system.
For dialing purposes, however, the full number is still dialed regardless of the dash ‘-.’