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.
A brief guide to legislation for trees
A brief guide to tree work terminology and definitions
Can I get my neighbour to cut back or reduce the height of their trees or hedge?
Help for Arborists
How can I find out if the trees within my ownership are protected?
How close can I build to my tree?
How do I select the right Tree Surgeon?
When is the bird nest season?
Construction and Trees Guidance
Directory of AA Registered Consultants
Directory of ARB Approved Contractors
Guide to Tree Pruning
Guide to Trees and the Law
Help for Tree Owners
Trees and Wildlife Guidance
Tree Selection, Planting and Maintenance
Tree Surgeons and Tree Advisors – Guidance