Actually /a.plɛ/ as a phonemic representation is not really correct. It's a contraction that people make, typically depending on their accent (mostly north part of France I'd say, but nearly everyone makes it sometimes) or speed.
If you are speaking without contraction, appe- is pronounced /a.pə/ or /a.pɛ/, depending on the sound that comes after the 'L'. IIRC ə can be silent, which covers the contraction case.
If there are no sound after the 'L' it's /a.pɛl/ (and written with 2 'L').