What determines the limit on voicemail storage on an iPhone 7?

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.

Whoever you spoke to one the phone was wrong, your carrier limits the number of voice mails you can have stored.