A 'phone' can be any type of phone, either fixed or portable. You know from the context what type it is. If it's on a desk in an office, you know that it's a 'landline' telephone. But if someone says 'Oh no, I left my phone in the taxi!' you know that it's a portable phone, which is known as a 'mobile' in the UK and a 'cell' or 'cellphone' in the US. I'd be interested to know what term is used in Australia, New Zealand, S. Africa and other English-speaking regions.