My iPhone 7 stopped taking voicemails. I dialed it from a different number and got the automated message, "voice mailbox is full." I contacted my carrier, Credo Mobile, and they explained that voicemail storage occurs on the iPhone, not in my carrier account.
On the iPhone 7, I went into Settings, General, iPhone Storage, Phone, and saw that Voicemails took up only 12.3MB. (This was after I had deleted some voicemails and was again able to leave a message.)
Is there, then, some constant, fixed, number of bytes that my phone can store as voicemail? Or is it a fixed number of messages? Or if I reduce the amount stored by some other app---say, Voice Memos, which today uses a whopping 14.86GB---will there be more room for voicemails?
These are not several questions, it is a single question with several possible suggested answers.
(Because of spam phone calls and robocalls, there is a steady drizzle of unwanted voicemails into my iPhone 7 so that even if no one calls me that I care about, the voice mailbox will get full.)
Note that one response to Can I devote more room to my voicemails? states, in the context of Verizon,
Voicemail is handled by the carrier. The size of your mailbox depends on the plan & features you have with Verizon, not on the free space available on your phone.
But Credo Mobile customer service---a live human being speaking with me---unequivocally stated the opposite for their service. In addition, when I logged into my account on Credo Mobile, https://www.credomobile.com/customer/account/login, I found no option to set or change the amount of voicemail storage. Also, there are no voicemails in my online account. For instance, I cannot go online and listen to voicemails. There is no list of voicemails online.