Hi,
I have a few clients I handle calls for when their full-time virtual assistant or PA is off on holiday. [I don't offer a full-time call handling service as it means being tied to the desk all day -- that's not what I started a freelance business to do

]
I use a VoIP PBX service,
www.voipfone.co.uk, and have set up a couple of numbers on there that I use for these clients.
I use a Snom 360 VoIP phone which has four lines on it (and you can extend this to 20 or so). The Voipfone service comes with voicemail to email, you can set up business hours, and divert your call to your mobile if you're going out. You can also have it back up to your normal landline number if any service issues arise or your broadband is down for some reason.
If your client also uses voipfone, you can set up a line on the Snom phone to show a busy lamp when your client is on the phone -- I've found that very handy when the job includes transferring calls to the client.
Cheers,
Tracy