Even with a phone call, your number will be logged and what you say will be reason enough to log your number. It's not like you're calling a wrong number, it's a BP ad that is purposely posted to catch guys looking to purchase sex from ladies, usually in an underage sting operation.
How is it harder to prove when you call, you still have to communicate in some fashion. Just the fact that you are calling a bp number from an escort ad shows intent and an opening of communication for the purposes of purchasing. Asking her her availability adds to this. Unless you're incoherent and mumbling so that they end up hanging up on you, you're safe. Otherwise you'll be logged unless you block your number.
Not saying you're wrong, and unfortunately, not sure if there are many C-36 cases to learn from, but I think texts or calls that are vague might be incriminating IF the ad is highly suggestive or explicit, such that there's little or no chance of arguing you were just looking for companionship or massage and not sex.