Choose the correct form of the verb. There ___ a lot of calls on my phone yesterday.

The correct answer is "were".

Here's why:

* **Yesterday** indicates that the sentence is in the past tense.
* "A lot of calls" is plural, so we need a plural past tense verb.
* The past tense of "to be" that agrees with a plural noun is "were".

Therefore, the correct sentence is: "There were a lot of calls on my phone yesterday."