Ideally you'll use a different phone number for each location, but it is a common situation and Google is getting better at dealing with this.
The primary risk is that listings will merge where you don't want them to.
Another risk is that you could end up with two listings for each location, one with the 800 and one with the local #. So, you want to try and find all the listings in the ecosystem with the local number and update them to 800 number.
If you can convince the business to use a local number for each listing, that would be much better.
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