I don't provide phone services as a general rule but do screen messages (very rarely) for clients who are on holiday.
In those cases I purchase a Skype-In line local to my client (comes with free voicemail) and then my client forwards their calls to that new number for the duration of their holiday.
I check the messages every few hours - email anything urgent or follow any other specific client requests, sometimes getting back to people (usually by email).

Here's another thread on the same topic that you might find helpful.

If you're going to provide this service you'll have a corner of the market cut out for you, as Mary said - it's a rarely offered service.

__________________