Hi CustardMcDoodle9312!
Does this issue happen only on specific call number or all number?
If a user attempts to retrieve a call and is unsuccessful, you can try to check the following tips:
*Verify that the user is using the Teams client or a Teams-enabled device/Phone.
*Grouping- is the user a member of the call park group, which is the based on having the same Teams Call Park policy assigned.
*Island mode- Call park and retrieve is unavailable in Teams island mode.
*The call has already been retrieved or terminated