you have to narrow down the problem a bit more. The given information is way to general for anyone to be able to help you.
when you talk about exchange online with 365 i would assume you have a standard M365 E3 Licence, If that's the case you may use Office simultaneously on 5 devices (3 PCS and "a few" phones is more or less than 3 phones?)
more infos here https://support.microsoft.com/en-us/office/how-sign-in-works-in-microsoft-365-1d646e83-1585-4278-8daf-d4a2cc0905e0#:~:text=With%20Microsoft%20365%2C%20you%20can,Macs%2C%20tablets%2C%20and%20phones.
Exchange Online + Office 365
one of my cutomers has rented exchange online with O365. He has Problems with his email accounts.
He has configured those mail accounts on many different systems.
3 PCs and a few mobile devices.
Its 3 PCs with 3 mailboxes and 3 Phones/iPads. Each of them has all three mailboxes configured
Now Outlook on some clients can not access the mailbox and displays a waring that the connection could not be established.
I assume that MS has a hardcoded connection limit inside their exchange?
I can not get a hold of support by telefon...
So how do i fix this now?
This is one of the messages i get trying to sync the mailbox:
If i try to open the mailbox within outlook i get a message that the mailbox can not be opened becuase there is no premission to open it
Sometimes this works tho ...
I'll collect another screenshot of this when i speak with my customer