You do not need to contact the pevious tenants. Just sign up with your provider of choice and leave them to sort it out, or, if you want different line and broadband providers, sort the line out first then the broadband.
One would presume that the prior tenants instructed cancellation of the services, so that may not have been processed yet. It will need to be, and the current service ceased, before you can have service on the same physical line.
if the prior tenants were evicted/ran off/forgot to cancel then there would be a delay provisioning on the same physical line but if you have two, there's a way around that.