Separable and inseparable phrasal verbs carry different stress. Separable phrasal verbs have stress on the particle of the verb. Inseparable phrasal verbs, however, have stress on the verb.
The separable phrasal verb pick up has stress on the particle up. In the sentence She picked up the phone the particle up is stressed but the verb picked is not.
Compare this with the inseparable phrasal verb look at. In the sentence She looked at her watch the verb looked is stressed but the particle at is not.
One key difference between separable and inseparable phrasal verbs is that pronouns come between the verb and particle in separable phrasal verbs (Pick it up!) and follow the verb and particle in inseparable phrasal verbs (Look at it!) However, another key difference is stress. Separable phrasal verbs have stress on the particle, but inseparable phrasal verbs have stress on the verb.