The answer to this is going to be different for different sets of parents. Not all parents who give their teens cell phones will be planning to give them more freedom. My own view (my oldest daughter is in 7th grade and is therefore not yet a teen) is that giving teens cell phones should allow parents to give them more freedom as well.
The main reason that a parent would be worried about letting their teen go out with friends is because they would worry about not knowing where the teen was. They would also worry about being unable to contact the teen if needed and about the teen being unable to contact them. If parents give their kids cell phones, these worries vanish. The parents can be much less concerned. They can know that their teens will always be able to contact them. They will also know that they can always contact their teens if they feel the need to check up on them. For example, a parent might set a rule that the teen must always answer their phone if they see that it is their parents calling. If the parents would always be able to call, the teens might be much less willing to try things like going to drunken parties for fear that their parents would call and they would be unable to explain the noise.
In other words, cell phones give parents a great deal more ability to keep tabs on their teens while the teens are out. This makes it much more likely (in my view) that parents will allow teens with phones to go out with friends.