flashing to metro and using custom roms

[Randy954]

My question is, is when metro pcs or other cdma carriers flash a phone to their network is it the same as flashing a custom rom? Or do they just put it on their network and then I can go ahead and use any rom i want (well thats compatible with metropcs network)?

[dfrancisjr]

if you already have metro you can use the cab file attached and in connections input your 10digitnumber@mymetropcs.com password metropcs

[Randy954]

Ok, but that doesnt really answer my question. Once i take my phone to metro and they flash it can I reflash it with any rom of my choice or do I need to use the one they give me? Because im thinking they do a different sort of flash just to put it on theyre network and then i can go ahead and use lets say juicy's rom?
Am i right?