It has a pattern lock on it, and my mother doesn't know the pattern.
She says that the only reason I'd put a pattern lock was if I was doing something wrong. I say that I'm not, but then she insists that I remove the lock.
Depending on your age and the purpose of you having a phone (some parents of children of 10-15 do it more for "personal safety" than anything) I would say there's a workaround to this.
Assuming that you don't have anything on there that you think your Mother would find questionable, offensive or would disapprove of- why not let her know what the pattern is, or change it to a PIN that she also knows?
This gives you two benefits- firstly for security's sake, nobody but you and her know the code, secondly it lets her access it if she really wants to so it eases her mind.
However there are two other important factors here- firstly, is it your phone, as in bought with your own money?
If it's your phone that you have bought, with your own funds (or paid monthly, however it works out)- it's your phone, plain and simple. Your mother has no real say over you having it, but obviously it's causing friction- was this why it was taken before? Depending on where you live, it's worth noting that some mobile carriers won't enter into a contract with you depending on your age- so arguably there's a legal issue here as well- as if your parents pay for it, well, it's entirely down to them.
Secondly if your age is in the region of 15+, this enters into the territory of just plain invading your privacy.
In terms of age, if 15+ we need to start talking privacy. All parents worry about their children and who they're talking to- depending on gender as well, Girls tend to be even more worried about.
The best you can do is be fair- depending on the above: either try and work with her by letting her have acccess if it isn't paid for by you etc, or if it is yours etc then she needs to accept you're growing up and that you need to be your own person, with your own responsibilities and to be trusted.
Unless there's a good reason why she's taken it before and is apparently quite controlling over it, this shouldn't be a problem- but at the end of the day this is your mother and presumably you live at home with her, so it's "her rules" unless you're old enough to be paying rent and to be considered a young adult.
Perhaps it would be worth your trying to have a reasonable conversation with her and explain why you have the lock on, that it's due to siblings and strangers- is this a similar situation to your siblings as well with their phones?
Discuss trust, giving you responsibility etc and that you're old enough to be given the chance to be an adult as unless there's a good reason for you having it taken off you, it's fair to say you deserve a chance.
It's all very well to say that as parents they have XYZ rights- but being a parent isn't always as simple as a black and white view, especially as kids age you have to allow for a certain margin, with "wiggle room". You're a teenager, it's widely accepted this is a period where people start to begin to "find" themselves and it's tumultous at times. They may percieve what you're doing as bucking authority, but what- do they expect to beat you down the instant they disagree with you on any point?
A time comes when some freedom is something they'll need to give you- they may not like it, you don't like things at the moment- it will either come at the point you move out in the future, or sooner if you can both come to an agreement.