Hi @Arham Kamal ,
Are you trying to add a phone number to your Microsoft account? I did some searching and found that a user had a similar issue when he tried to update the phone number associated with his account using Mozilla Firefox. After getting the error message, he tried updating using Microsoft Edge, which worked fine.
Besides, your issue is more related to account, please kindly understand here which mainly focus on general issues about Outlook desktop client. It is suggested for you to post a new thread to the dedicated forum via this link, so that more developer experts can provide you with more targeted help.
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