They are contractors. They don't work for Google. They either work for themselves and have a contract directly with Google, are an employee of a company which has a contract with Google or are a sub-contractor to a company with a contract with Google.
If they are directly contracted with Google or a sub-contractor through another company they should purchase an additional phone for this purpose. Both the phone and the service would be considered a business expense for tax purposes.
If they are a direct employee of another company then that company should be providing a phone for this purpose. If Google or their employer won't provide a phone for this purpose than neither considers it a requirement for the job and they should not worry about it.
If they are directly contracted with Google or a sub-contractor through another company they should purchase an additional phone for this purpose. Both the phone and the service would be considered a business expense for tax purposes.
If they are a direct employee of another company then that company should be providing a phone for this purpose. If Google or their employer won't provide a phone for this purpose than neither considers it a requirement for the job and they should not worry about it.