There are many ways to do this. Asterisk and a rented VoIP line, a hosted PBX service you just rent and transfer your line to, a 'Web 2.0' variation of the same

There are many ways to do this. Asterisk and a rented VoIP line, a hosted PBX service you just rent and transfer your line to, a 'Web 2.0' variation of the same (like Twilio), or even a programmable desk phone.