I was making an enquiry for my house the other day and was offered a small exchange for around 400 euro. Not sure of its capabilities though whether or not it plays music etc., it did not suit me. The company is in Sandyford in Dublin and if you google them you should find them, the name is Instacom.
Re: Small simple phone system for start up company 4-6 people/extensions maximum
depending on your circumstances a VOIP solution might be the answer to reduce installation and running costs. I saw a place down in the Digital Hub in Dublin, [broken link removed]who appear to provide phone services to small businesses including extension numbers, call answering etc.
To take advantage of such a service you would need reliable broadband.
I would look at Asterisk VoIP phone systems. You would only need to buy the handsets and install the free OS onto a spare pc running linux. The phones can be PoE which means your existing Ethernet can carry pc/phone. No outlay on wiring etc.
Its a bit more work from your end, but will save you a fortune in the end. You can have voicemail, IVR menus and call centre type features.
They sell a complete "appliance" but this is going to be more expensive.