Don't know the procedures where you are, but I would not be changing frequency without a prompt from the controller who is working you.
If in doubt, ask him/her, as you're approaching the CTR boundary.
Don't just call the tower without checking it's ok. Co-ordination between ATC units in respect of "shared" airspace or traffic goes on behind the scenes, either verbally or electronically, and you'd be surprised at how time consuming it is.
Calling one ATC unit while being controlled by another could throw a spanner in the works.