how do I retreive mailbox number on voicemail on my iphone SE

Sounds like your carrier does not support Visual Voicemail. Differences in carriers. Either way, the carrier is the one that can answer all questions about voicemail access, the resetting of passwords, and upgrading your account from regular voicemail to VVM.

It should be done automatically. Shouldn't it? If he uses an Android he can get the number. On my provider I can dial a 5 digit number to listen to the last saved message in my mailbox. All my voicemail goes to a number in Ottawa, Ontario.

It would depend on the carrier. The iPhone uses Visual Voicemail on most carriers and the phone does not call a number. The carrier's server contacts the phone and downloads the voicemail. It does not require intervention by the user, until they wish to listen, delete, or otherwise work with the message. If the carrier does not support VVM, then the carrier would need to provide them with the number to call for voicemail.

Contact your carrier. Voicemail is a carrier function. They will be able to recover it or reset your password.

All mine says is "call voicemail". That's it. When I had an Android, it would display the number being called. On my iPhone, it doesn't.

That's the way it works eh? Yes I'm Canadian. Lol.