Reading the ritual spells session on the game book I could not figure out the utility of Phase Door. It seems too weak for the amount of time and resources required to cast it. My first impression was that the spell was actually a form of higher teleport (a way to open a magic portal do cross any distance with safety), but upon reading again I noticed that it isn't, it is just a way to directly pass an immediate obstacle, like a wall. I can only imagine it being useful for entering a sealed magic temple/door/prison, but it wouldn't be possible to cast it to escape this same place though... unless you carried with you all the stuff and food you would need to use for casting the ritual again.
So, how exactly can I use Phase Portal?
The door has a duration of 1 passage per 2 levels, so you don't have to recast it every time you want to travel through.
I think Aryxymaraki answered this correctly so I will just stand here and look wise.
Also, when you perform a ritual spell you can opt to store the spell as a charge in a ring or similar item, so you could potentially cast a bunch of phase door rituals in advance and use them when you need them.