Go to the Lead Database in Marketo - ensure that you have access to all the lead partitions in the database.
Create a Smart List in either the default workspace (which should have access to all the lead partitions) OR Create a Smart List in each workspace that has access to the specific lead partition.
You would create your Smart List as follows:
I would definitely ensure that you are looking at records in Marketo that do not have an email address since you will not be able to market to these record (unless you are using them for SMS push notifications) in that case I would include Mobile is not empty
Hope this helps