Why is the call button greyed out.

The header colour alone does not explain why calling is unavailable, and your screenshot does not establish whether this affects one conversation or every call. Start a new one-to-one chat with the colleague's correct work account, then try another internal colleague. Repeat the test in Teams on the web using the same account. If calling fails for everyone in both clients, ask your administrator to check your assigned Teams calling policy, particularly Make private calls, and the tenant's service health. Microsoft documents that this policy can prevent users from starting private calls or escalating chats to calls. If only this colleague is affected, compare both users' ability to call others and confirm whether either identity is external. Ordinary Teams-to-Teams calls should not be confused with telephone-network calling requirements. Give your administrator the exact error, failure time, client version, and those test results before changing policies.