You definitely need it. How else are reception going to get you to go and ask Emily in Mr so and so's class that she has a dentist appt and needs to be down in reception? Several times a day sometimes! Or "can you ask Mr so and so that he needs to do his register"!!
I was going to post something very similar. To be fair we had an upgrade last year and phones went in all the labs, but if a teacher is mid sentence then they won’t answer them.
Also as part of the upgrade our outside line was taken out as they didn’t realise we needed it! Soon put IT right on that point.