Since you took on freelancing in order to have freedom, then I would not surrender that freedom for a marginal financial bump. I would tell the client you are

Since you took on freelancing in order to have freedom, then I would not surrender that freedom for a marginal financial bump. I would tell the client you are available by phone or email during business hours, and that you will answer the phone, but you might not be at your desk. If you have a lot of conflicting meetings (ie. other clients) then lay out specific hours of availability to at least answer the phone. I feel the client would be assured if you just answered the phone when needed during business, but you don't necessarily have to be sitting at your desk when the phone rings. I would try this arrangement as best effort, and avoid hard-coding your hours into a contract, and as long as the client doesn't complain that you haven't been answering the phone then all is well.