You're using a custom template, so you need to ensure that the lookup scenario returns the following:
1) At least one of FirstName, LastName or Company outputs with a value set.
2) The phone number being searched should be present as an output.
3) The ContactUrl output should be set to a unique value.
If any of these conditions are not met, the returned contact is discarded and you will not see a popup.
But what's weird is that this has not changed from v18 to v20, so if it worked in v18 it should work on v20 too. Maybe you're changing the contact lookup number length setting? Check that from Advanced > Contacts > Options.