You are not out of luck. Generally, what is written down is all that matters. But you can still show by clear and convincing evidence that the writing does not conform to what the parties agreed upon. In addition, you may have two other outs depending on the lease's terms.
Does the lease prohibit modification except in writing? If not, then it was modified in your telephone calls.
If the lease says "one year" but also says it expires April 30, then it is ambiguous, and you can show that you meant it to end in March, not April. It would be helpful to have a document in which the property manager agreed that it ended in March. The telephone calls help, but the property manager may insist that they never took place.