I know it can be very difficult trying to teach when mobile phones are ringing. It's also very distracting for the students when they're trying to concentrate. If they're speaking, they can lose track of what they were saying.
First of all, you need to speak to the students about the noise and disruption. Explain that you find it difficult to work in such an environment and ask them what they think. They'll probably agree with you; or at least some of them will. Even if they don't find it disturbing, insist on the fact that you do. Then explain with a smile, that it's better for them to have a teacher who's giving 100% attention to them.
Get their agreement that it's necessary to switch off their phones when entering the classroom. Then at the beginning of every lesson, ask whether they have switched them off and allow them the time to do so.
Remember that students have private lives and may be waiting for an important call. If this is the case, they can tell you in advance and you can allow them a break to check their messages. You could also offer them the possibility of leaving their phones with someone at reception who can take a message for them.
If you handle the issue respectfully and stay firm about what you want, it should all go well.