I think this would depend on the type of lesson you were doing. For example if the teacher wanted to show you some text or some notes they were writing or a photograph to discuss, then you might find that the the phone screen was too small to be helpful. On the other hand, if all you want is is simple conversation, and you don't need to read anything during the lesson, then a mobile phone could be sufficient. Also I think it would depend on your level of language. If you were more advanced, then you would maybe need less support in the form of written examples, text, etc.

Hello! You mean is it okay to use your phone during your lessons or is it okay to use your phone as a tool on you lessons? As far as I know you can use your phone to have lessons with teachers, but most of them don`t compliment students if they reach for their smartphones ;)

You can and some are occasionally forced to use phones, but not in interactive classes where annotation is required. If there's an 'emergency' phone lesson I simply change the content to accommodate it.