Using a mobile phone up a tree during an AA Approved Contractor assessment then would not in itself result in failure unless a clear and present danger was apparent as a consequence. Arborists do use phones up trees and, whilst it may not always be ideal, there may be justified reasons to do so,provided that you are in a safe position and appropriately attached to the tree. Many companies have in-house rules and guidance for mobile phone usage (e.g. personal calls only at lunchtime etc.), this would be something you would need to discuss with your boss.
We have more help for arborists here with legislation and guidance.