after the shift, the positive charge occurs on phenyl which should be unstable.
Are you sure about that? Firstly, the positive charge isn't 'on' the phenyl, it's on the Carbon to which the phenyl ring is attached and hence it enables resonance.
Out of many of the factors we look for the stability of a carbocation (which include the ones you mentioned like positive inductive effect in a tertiary Carbocation, Hyperconjugation, )
Resonance takes a major chunk of the competition along with hyperconjugation of the neighbouring Carbons which is what Oscar pointed out as well.
In general, the Carbocation formed is highly stabilised due to resonance with the whole phenyl ring so there's no problem in forming such a carbocation.
When you'll try and draw the resonating structures, you'd realise that the delocalisation of the charge throughout the ring stabilises it.
- Hyperconjugation with 3 alpha Hydrogens
- Tertiary Carbocation
All these factors ensure high stability of this carbocation