When you say "keep a phone line" are you referring to a phone line with an associated number so they can move in and start making calls?
If so, I do not think that is a good idea? The phone will be under your name, regardless if the tenant is in a renting contract. It is like having utilities connected, they should all be under the tenant so they have the responsibility.
What is to stop the tenant from racking up an enormous phone bill and simply not paying it?
If however you are just referring to having the connection available for the tenant so they can arrange their own line then this is just standard. There are exceptions to this such as when i rented in a block of 6 townhouses. I tried to get internet connected and was informed only 3 lines were ran to service all 6 properties. Now because they had already been used by other tenants I had to pay for a new connection ($300!). I thought the developer was required to run a line for each property, but turns out they don't??
Same goes with internet. I personally would not want a tenant using internet that was in my name. Who knows what weird and wonderful things they are going to be looking at and doing.
I guess what I am saying in summary is, YES most definitely a phone line "connection" should be available to your property, BUT if the tenant wants phone or internet then it should be their responsibility to organise it. Exactly like they do with water and electricity.