Based on the information you have provided, it would appear that you are carrying on a rental/leasing activity that would require you to complete a rental schedule (T776) and where you would be able to claim, as an expense against the lease payments you have received, depreciation (CCA) on the cost to you of the communications tower.
The communications tower would fall under the following CCA class:
Class 46 (30%)
Include in Class 46 with a CCA rate of 30% data network infrastructure equipment and systems software for that equipment if they were acquired after March 22, 2004. If they were acquired before March 23, 2004, include them in Class 8.