I hired hourly employees, they work remotely and choose when to work by themselves.
The work includes working on a computer + making phone calls.
How do I check the actual number of hours they spent?
This is always an X/Y problem. It is almost always an attempt to address systemic organizational failures by "holding employees accountable." Any process or technical control you can apply to tracking employees, rather than tracking productivity directly, is (at best) a proxy for failing to address the underlying issue: you are tracking a proxy metric for productivity, rather than tracking results directly.
Building process controls and tracking metrics around results can be hard. However, all jobs should have a value proposition that can be tracked in a quantitative way.
For example, your remote employees make phone calls. You don't say what the calls are for, so we'll assume some basic things:
A good results-based metric wouldn't look at call volume or call length, as they are (usually) proxy metrics for productivity anyway. Instead, you should establish a baseline, and then track the organizational benefit that good call-handling is supposes to produce.
For example, let's imagine that the calls are technical support calls. These calls don't actually generate revenue. Heck, the entire technical support department might be considered a cost center! However, good technical support prevents product returns or lost customers, so there is a clear organizational benefit to the activity.
Given the above, if customer retention or product returns remain the same or better, then your team is doing the job the organization needs them to do. If you lose customers due to poor technical support, or you have a spike in product returns that could have been resolved through technical support, then the team is not doing their jobs.
If you can't identify the organizational benefit, or are unable to track the benefit, then fix that instead. Tracking the wrong metric or incentivizing the wrong behavior is often worse than not tracking productivity at all. Instead, you will be training your team to look busy, or game the system, without any meaningful benefit to the organization.
You don't want to be the person who creates a culture of busywork and unproductive behavior at your company. That's the fastest track to the unemployment line that I know of, and treating your employees as presumptively lazy or incompetent is just greasing the skids. Don't be that person!
There is only one answer to this question, and I am sure you know it, you have to ask your employees to record the time they are spending and then monitor your records. There is no stealthy magic process that will inform you of what they are really doing.
These kinds of questions are usually asked because the employer believes the employees may be slacking and billing for time they did not spend. First consider if there is any real evidence of this or whether you are just being paranoid. If the work is getting done in roughly expected timescales then be happy and don't try to micro-manage every employee's working hour- it is a counter-productive exercise, you will not win.
If there is evidence that you are being ripped off then you are in a difficult position. Unless you deploy some computer-based controls for remote workers you have to just trust what they are saying, there is no other way to know. If you cannot do that, then you will need to bring the employees into a controlled environment- i.e. cease allowing remote working and force everyone into the office where slacking is easier to detect and deal with.
It's possible to setup a system such that your devs need to work on a server you own, and that logs their activity... but that's probably more trouble than it's worth, and it would likely hinder productivity more than help. Beyond that, it would probably take a lot of your time to take whatever data you've gathered and tie it back to what they said they were doing at any given time, double-checking their hours.
Ultimately though, if you're not going to track all of their activity, you'll need to take them at their word. I've had remote developers take advantage of the situation, and it sucks.
However, if that's the case, eventually one of two things will happen:
Generally, when I'm managing remote developers, if they get their stuff done on time and they stay on budget -- I actually don't care a whole lot if I know exactly how much time they're spending. What matters are results.