OK, maybe I have been scarred by the schools I've worked in, but the telephone facilities in most UK secondary schools appear to be rooted in the 1960s. My preference is therefore to buy a mobile phone which I can use from any empty classroom.
Here is my advice:
1. Cheap, cheap, cheap.
Buy the most basic Pay-as-You-Go phone (under £20). Put £5 worth of credit on, no more. These phones are surprisingly useful - for one thing, they very rarely get stolen.
2. Incoming Calls
Do not use your phone for incoming calls. Set up your answerphone to say 'Thank you for calling <your school's name>. Unfortunately, this phone does not take incoming calls. Please call the school office on 01234 567 890'. If your phone rings while you are using it, don't answer - terminate the call. Make sure that the phone's Call Register is enabled, so you have a record of all incoming calls.
3. Incoming Texts
Reply to all incoming texts with a message: 'This is an automated response. This number does not accept incoming text messages. Please contact the school office on 01234 567 890'. Do not delete the incoming text. Keep a complete record of all texts in your Inbox.
4. Outgoing Texts
Do not send outgoing text messages.
5. Outgoing Calls
Use your phone only for outgoing voice calls from a pre-written list in your notebook. Your notebook page should routinely have a date on it, and you should therefore have a complete record of who you phoned when and why. Make sure your phone's call register is enabled, so you also have a physical record of all calls.