Today there are call outside job expectations, tomorrow they will worry you are not following up in a couple of minutes with work emails in your private time...(being sarcastic here)...Getting to business:
Given strong European laws protecting employees, I would talk with an Union attorney/conselling person about this.
I would undertand giving phones to a mobile force (e.g. a consulting/services) firm or to higher ups (upper management), and mainly to call them during business hours.
Giving you a phone, or even having your private mobile number should not equate for them to call as they please.
Usually businesses that need on call persorns do a roster schedule of one or two weeks per each employee, and rotate the work phone or voIP number between them. That means extra pay, a regular pay for being on call, and at least 3 hours pay and then hourly just for being called.
In the past, when I was the IT director of an ISP, I was in the unfortunate position of having a phone number known between 2 and 4 thousand people, and if I did not manage when to pick up a call I would have gone crazy.
I had several strategies for when called out from not picking up calls, ranging from "I was sleeping/walking/on the beach/making sex", and I was paid enough well to pick up the phone, however at the end of the day, it is your life and your personal time.
I also put it in silent mode when taking care of sensitive malfunctions where I needed my cool and thinking on my feet, and let my help desk crew pick up the calls.
People can be also rude, and think that giving you a phone, you are at their beck and call...I do remember a customer from whom I had never heard of ringing me 10 times in a Saturday night at 11PM, and still managing to complaint I obviously did not pick up the call. Clue in, having a phone does not means you have to pick it up to whoever decides to call you.
I usually establish some boundaries;
- emails for not so urgent matters that will be read in a best effort basis;
- google chat for out of band communications with a small subset of trusted colleagues;
- SMSes for really urgent matters;
- phone calls for bad situations.
When I was a consultant, I had a company issued phone for 5 years, and people were pretty considerate enough to not call after hours; as a perk, they also allowed us to make calls with a monthly ceiling 100-150 Euros.
As for "security" reasons, unless you are a spy, or work for a top-secret government agency, or are a member of parliament, I cannot really get as that will fly as an excuse for issuing employees a mobile phone; it seems more an excuse to circumvent current trends on EC about employees not being obliged to keep up with work email outside work.
As other comments also correctly point out, a locked down smart mobile phone with a company issued account can be pretty much abused as a tracking device especially while it is on and connected to the Internet in real time, and it will have your whereabouts in the history, and that also has to be correctly managed.
Again, ask the advice of your local union representative; I suspect that you can pretty much say no to carry a company issued mobile phone outside work hours.
Having said that, there are probably countries where you will have more legal backing for this; if in Germany, I do strongly stress talking with a union; in southern countries few will bat a lid.