Invasive or am I being grumpy?

[TutiFrutti]

Hi, new to MN and first post.
Just looking for a bit of perspective although I don't think I'm overreacting.
I have a much loved relative that I talk on the phone with frequently, this relative lives with someone who is significant in their life but is no relation to me.
During our calls my relative has the phone on speaker (due to hearing loss) the other person has a habit of interjecting and correcting anything they perceive to be incorrect.
I usually deal with this by just pretending I didn't hear what they said and carrying on the conversation but it is very distracting.
I have tried explaining that I can't hear them both when they talk at the same time.
I have explained that I just want to talk with my relative in peace and at one point I confess I did slam the phone down when this person snatched the phone out of my relatives hand and started shouting at me because they didn't agree with what I was saying.
How on earth can I get the message through? This person is nice enough but is seemingly unable to respect our privacy.
I should add their behaviour seems to stem from huge insecurity but while I have no wish to make this worse, surely I should be able to speak freely to my own family?

How does your relative react when this happens? Do they seem upset or laugh it off? How long has this person being around your relative as they sound very controlling. Ultimately it is up to your relative to put a stop to it, theres really not much you can do from your end,except not call, which obviously you dont want to do.

@AlicebytheSea thanks for replying, you've pretty much hit the nail on the head, this person has displayed other controlling behaviours. They have been on the scene for a few years now, I have expressed my concerns to my relative who does get annoyed with this person sometimes but ultimately opts for a quiet life.
On the whole I let them get on with it. I refuse to stop calling though as I feel it's important for my relative, I'd miss them if I didn't and basically I'm a mardy old cow!

What would s/he say if you said " oh I'm just talking to eg my grandma now give us 5 minutes then I'll have a chat with you?"

Would headphones help your relative maybe?

@AlicebytheSea I've tried that, been polite and less polite, stops for a while then gradually starts up again.
@BusterTheBulldog that's not a bad idea!

YANBU
If I were you I would ‘rinse and repeat’ as is often suggested on MN with people like this. How about something like ‘I am talking to my relative not you, please stop interrupting our call, I can’t hear my relative and they can’t hear me if you constantly takeover our conversation’.

When the behaviour starts up again just repeat the same sort of message every time. Yes, it will be tedious but hopefully they will butt out for a while at least until you need to reinforce it again. Your relationship is important to both of you so don’t let this person think they can walk all over you both.

@TooTiredButItsChristmasAargh
To be fair this has been going on a long time and I've been doing exactly that but still it goes on. I'm at the point where I make my excuses and leave the call when it happens, just makes me sad my relationship with my relative is so stilted now.
Occasionally we chat when the other person isn't there and it's lovely. I miss that.