[twicksisted]

I have an online retail business which I've been growing for the last two years. Im in the UK and the freephone number prefix for customers to call us is 0800 which I can port to any phone number/system.
I want to build a computer/server/nas that I can install software on to run the telephones.
What im looking for is a business system when a customer calls they get to choose the department they want to speak to and go into a queue if no one is available.

Currently we are pretty small and it will be for two internal telephones (two sales people)... but as we grow this would expand and I'd like to have functionality for when this happens and not to have to migrate to another system.

Any reccomendations out there? Cost is king (If theres something that we can use that dosent have a monthly payment and we can buy the license to outright that would be perfect).