(Note: Unable to find a picture for it yet. Contributions are welcome).
Observations: At first glance, the phoenix doesn't seem to have much going for it. Its main tactic seems to be to immolate itself every round, dealing fire damage to everyone and healing itself of all damage. This effectively puts the players on a 1-turn clock. They have to defeat it within that time, or all their efforts will have been for naught as the phoenix 'refreshes' itself. Though with a casting time of 1 full round, the players can just move out of range, and 15-ft radius isn't really that wide.
However, this isn't as tough or daunting a feat as it may sound, given its pitiful 150hp and negligible 23AC. A fighter can't help but full-attack for much more, and a wizard can easily one-shot it with a metamagick'ed damage spell. The players are never going to let it cast for 10 rounds uninterrupted, and let's be honest, what's the point of dispel magic at caster lv40?
As was the norm for many higher lv monsters in 3e, the phoenix received a glut of low lv SLAs it will likely never use in combat, but are flavourful to have. Why bother with CLW when it already has heal at-will? Indeed, most of them are redundant in combat, and generally inferior to just exploding every round, making the phoenix somewhat of a 1-trick pony.
Its slowing screech can be effective as slow is something that is almost impossible to counter, barring haste. Again, don't bother attacking in melee.
Interestingly, the phoenix is not immune to fire.
Remember that with the 3.5 update, it gets dr 15/magic cold iron and maxes out 6 skills.
Recommendation: I am going to cheat a little here by assuming defensive aura applies cha mod to AC instead of a flat +5, and apply the monster of legend template (MM2) to the phoenix. My reason is that its base stats are way too weak for a high-cr entity, yet its signature ability is too powerful for a mid-cr foe. It's basically a quick and efficient way of improving its stats across the board. This changes its type to outsider, increases the phoenix's hp to 250, AC to 33, an additional skill to max out, and saves are less anemic (Fort+22, Ref+21, Will+12). For special abilities, you are free to customise; I went for frightful presence (the rest don't seem thematically fitting), enhanced attributes (+4DC to abilities) and immunities (mind-affecting/polymorph, as it will save just sucks).
Remember that with death ward at-will, it should never enter combat without one active. Final cr, I would peg at 18-20? This one, I need some feedback.