While a DSL is nowadays more important than a telephone, it is still necessary to have a properly functioning telephone connection in the apartment. A lot of service providers have packages for both telephone and internet for the same price (e.g. Deutsche Telekom). If you subscribe to one of these packages, remember to provide your tenant with access only to the flatrate options for within Germany. Other options for which you have to pay, such as international calls, cellphones, 01- and 09- numbers, should be blocked. Otherwise, you would be obliged to get an invoice for the telephone costs of every tenant.
Most of the tenants use Skype for internatioanl calls and don’t really need a teleophone for that. And most people nowadays have a cellphone anyway. Don’t forget to provide the tenant with your own cellphone number in case he or she needs to reach you in the case of an emergency.