If the system just dials a phone number how do you then open the gate? As well as complaining about not being consulted I would be asking about running costs.
Using a cheap PAYG phone sounds like a good option. An alternative would be to buy a landline phone that will use a different ring tone for different numbers. You can then set a ring tone for the gate and a different one for everyone els.
Can you get caller ID on your landline so you can recognise when the gate is calling you and answer?
We have a gated entry system that comes through to our landline and Ihate it. I can't be on the phone if I'm expecting a delivery, so can't get on with my day as normal, and sometimes the entry doesn't work properly (you hit # to open the gates) so I have to run down and sort it out. Horrid thing.
We have a system like this at the school I work at - when it rings you press the green button to talk to the person at the gate, 0 to open and then red to hang up. We have two handsets and it sometimes has problems if someone opens the gate and then forgets to hang up and the next person to answer uses the other handset - so you might find a problem if someone else hasn't hung up after opening the gate. It's generally pretty reliable, but then it's only been there a few months.
I have now saved the gate number to my mobile so if it balls up and doesn't open I can ring back from my mobile and get it to answer. We're also furthest from the gate so I feel your pain on that one!
It sounds from your OP that this is going to happen so being able to come up with ways round it is probably for the best. Phones with called ID really are quite cheap, and I expect you'll only need to keep an eye on it when you're expecting people.