Need iPhone 7 Voicemail Help

My mom has an iPhone 7 and is trying to delete her voicemails. She’s been trying to do this for months and every time her voicemails magically come back. Now it’s completely full so no one can leave her any voicemails (and she’s busy and gets a lot of calls so being able to get voicemails is very important for her. 

She’s tried the whole turn on airplane mode and delete while in airplane mode thing, and they have all still eventually come back. She’s searched the internet high and low for an answer.

Her current theory is that when her iPhone backs up to the cloud at night, it’s also backing up her voicemails and bringing them back. But we don’t know, that’s the theory. 

Can anyone help her? (With step by step instructions because we aren’t always very technologically smart ;P )

Thanks to whoever can help!

Need iPhone 7 Voicemail Help

My mom has an iPhone 7 and is trying to delete her voicemails. She’s been trying to do this for months and every time her voicemails magically come back. Now it’s completely full so no one can leave her any voicemails (and she’s busy and gets a lot of calls so being able to get voicemails is very important for her. 

She’s tried the whole turn on airplane mode and delete while in airplane mode thing, and they have all still eventually come back. She’s searched the internet high and low for an answer.

Her current theory is that when her iPhone backs up to the cloud at night, it’s also backing up her voicemails and bringing them back. But we don’t know, that’s the theory. 

Can anyone help her? (With step by step instructions because we aren’t always very technologically smart ;P )

Thanks to whoever can help!