A phoenix is a bird from Greek mythology. When a phoenix dies it burns up, it can then rise from its ashes once again. (See Harry Potter for a good example)
Simply, the shonky company shuts down (or goes bankrupt) making it impossible to for a customer to get follow up service. Often a new company is then formed by same people without their previous liabilities. Born of the sun, the phoenix company term is particularly apt for this shonky practice by solar companies.