She's 11? Middle school I guess and so girls are constantly texting, on instagram, snap-chatting, etc. When you say ont he phone I have to assume online, internet, games, etc. - not neccessarily talking on the phone?
You must limit the time. I assume her mom has the service and pays for it? If so then you have no control that way.
So - instead you have to set up a place where the phone needs to be during certain times of the day. Have a small basket or decorative glass bowl on the kitchen counter or some other central location - I'd give her no more three hours a day total - depending on how her schoolwork is doing, what other things she's involved in, etc. If she does homework in the afternoon then she gets an hour in the afternoon - AFTER homework is done. And an hour or two in the evening (depending on bedtime).
Engage her in terms of limits. Ask her what she thinks is the right amount of time. Start by saying we've concluded that there has a be a healty level of cell phone use - so we're going to set some limits. If you were the mom of an 11 yr old girl how many hours a day do you think would be about right? Kids somestimes surprise us by providing a better answer than we thought they would!
You also need to know what she's doing on the phone. The world wide web can be like thew wild, wild west. You don't know who is out there - and phone can often be set to track their location. On facebook kids will often accept anyone as a friend and they give away so much personal information that a predator can easily find these kids. If she's on FB you must be her friend as well so you can see what she's posting. Get her FB password too. I still tell my 18 yr old to delete a snarky or mean comment. she knows she has to do it as long as she's depending on us for financial concerns.
Our kids, even to this day at 15 & 18 know we have a random audit policy. At any given moment I can ask for their phone and look at their texts, who they've been calling or getting calls from, their online history, etc. There have been times when I've audited them frequently (when there's trouble going on) and once they show themselves to be honorable then the audits decrease. (In fact I haven't done that in a long time - I think today may be one of those days!)
Just because her mom pays for the service doesn't mean her dad has no control over what she does on the phone. Think about it - if her mom gave her a car for her 16th birthday would she be allowed to drive to Mexico or NYC? I don't know what kind of relationship you, as step mom, have with her - so depending on how good your relationship is you need to figure out whether you or her dad get to be the "auditor".
We have a good relationship with our kids. They have chores, they do volunteer work, they have to go to church ativities and youth group (so to keep their church friendships alive) and they have to maintain decent grades - but we give them privileges that are age-appropriate and unless they do stupid things they get more privileges. Age 11 is when you begin to give them small privileges - which come tied to responsibilities. At age 11 she can vacuum, or mop the kitchen floor, she can load or unload the dishwasher, clean off the bathroom vanity, etc. Doing those things when asked, without complaining allow her cellphone privileges.
One of the positive things about her having a cellphone that she obviously loves having is that you have a tool to use as motivation or for discipline.
I know it's another thing for you as step-mom to deal with - but I can understand her mom wanted to give them a way to contact eachother without having to go through you. (The only kids I knew who had cell phones at a young age were kids who had a parent they didn't live with.)
You seem to be level-headed so I'm sure you can come up with a good policy. I'm often sstung when my kids call me out on my own policies !