You may do this however you want to, using the available tools, as long as you do not violate Upwork TOS.
In order to formulate your strategy, you need to understand that Upworl's tool is not a time clock. It is not designed to record precise time intervals that may be associated with short phone calls. Ideally, it is used by freelancers working in blocks of time of at least forty minutes in length.
It actually records time intervals of an average of ten minutes each... but the times that it takes screenshots are random.
So if you take very short calls through the day, you could end up undercharging.
It may make more sense to use manually logged time, and add up accumulative time segments recorded elsewhere. But a client would need to allow this.
You also need to take into account the value of your time spent waiting and not handing calls.
I would not accept a job in which I was expected to be "on call" doing nothing else eight hours a day. Unless I was compensated for that time as well. For example, the client could set up a weekly retainer payment of $XXX.00 dollars per week to secure my time during specified hours, and I bill time on top of that.
OR it is understood that my time is not exclusively reserved, and that I am one of a number of people who may be handling calls. Whichever one of us handles a call is paid for that time, but we are not expected to be ready to field calls all the time.
But am I going to agree to be available exclusively to handle this client's calls for nine hours a day five days a week for the possibility of working for thirty minutes a day?
That is unlikely, but you ARE allowed to do that. The Upwork user interface does not prevent you from doing that. The numbers may even work out. If you are paid enough per hour to make it worth your time, and the times not spent handling calls can be used for something you want to do and can interrupt easily enough... then go for it.
