Phones weren't "hacked". What was generally happening was that the baddies were exploiting a feature of the voicemail systems - the ability to dial into a voicemail from any phone, enter a PIN and get access to the messages. This feature allows people to listen to their messages even if they don't have their mobile with them.
At the time, most/all networks had a default PIN, which few people bothered to change. To listen to someone elses voicemail involved calling their mobile, waiting for it to divert to the voicemail then entering the PIN. Given most PINs were 0000, or 3333, or whatever was default for that network, it wasn't difficult to get access.
Since then a couple of things have changed:
- Networks no longer have default PINs - every one is different
- Some networks only allow you to change the PIN after calling from your own mobile, rather than allowing you to change it when dialling in from another phone.
- Some networks monitor attempts to access the mailbox remotely, and lock access if they detect failed remote access attempts.
It has nothing to do with the make of phone - voicemail is kept within the network (hey, it's a cloud service! Before the term was even invented!), not on the phone. (Exceptions to this are 3rd party voicemail services that may also send the voicemail to the handset in the form of an MP3 - e.g. Hullomail).
(Hope that all makes sense... I do this stuff for a living)