You are dealing with a player.

She loved you once and the beginning was probably explosively good/happy time for both of you.
But eventually, she came down, got comfortable, less effort. less intimacy. Bored taking things for granted.
Became more of a relationship; out of living comfort. Than ''True Love''

I can guarantee a test result if you will, but it takes balls and has consequences,
It provides proof of the things you don't want to see, (ask this: ''If you and I would switch phones right now, would I find something I wouldn't like to see?''
Leaves you in pieces but with the truth.
And is also kind of humble since your not literally taking her phone and running to the bathroom, lock it and read it all. (make sure the phone is unlocked, or you know her code)

OR you could go on, good hoping, well-wishing, but then you will have to accept that she is cheating.
Don't bring it up, play the game with her and don't get caught. Live out your lives ''together'' Banging some1 else from time to time.
If this really starts being the case I recommend going to a swingers club. Its the sole reason for its existence.
And it annihilates this problem very well.
Because once the jealousy parts, you will have nothing left to be jealous of.
As you might see this already requires for you both to jump a hurdle.
AND IT ALWAYS WILL COME UP after some time. This is the times we live in.
The culture we are.

See there is a third option here in your imagination BUT this is a fairy tale and will not happen, don't watch the movies too much.
The third option:
Where she actually stops having a genuine sexual and intimate interest in this ''friend''
And goes for the know familiar you.
There is nothing you can do. She is already over you.
WAY before she even started to do it. She knew the risks. She decided that she wasn't happy with you anymore.
And wants something new. But the security a man provides leaves her with only 1 option. LIE and say it will turn out alright.
That this is just a friend, oh lord tell me who hasn't heard of this BS.
She protects her phone vigorously, gets more upset the more your digging into her phone.
If you continue she will find new ways since you discovered the phone thing.
It might now be Facebook messenger or fuck it even letters hidden under rocks outside to keep contact.
These girls are smart, careless, ruthless. When they lost interest in their partner. She is a schemer. Potential Narcissist/Sociopath.
They feel like you failed to make them happy and they might feel cheated on too. Because they spend all this time while you were not what made her happy in the end.

Listen boy, I just found out I wrote myself my own cure for the situation im in.
It's FUCKING HARD to let go. Especially when you are the one still caring so goddamn much.
But be no fool. Her RIGHT feelings have gone way before she started the phone sex. She is already over you.
That is how they can be so cold, careless. Distant. Etc.

Also, don't be fooled by sweet talks I know you want to believe them.
But this is what being played is. She takes advantage of exploits in your emotional system. [SHES A WHORE, I HATE HER ALREADY]
But it is very fucking unlikely the relationship's fire will restore to what you want it to be.
Because right now that fire is burning with some1else.
And that isn't you for her. She's already gone mate.

YOU NEED TO PART.
YOU WILL NEVER FORGIVE YOURSELF, Cuz YES SHE ACTUALLY WILL GO DOWN THAT FAR INTO MARRYING YOU. JUST FOR SAFETY.
FOR SELF INTEREST ABOVE ANYTHING ELSE.
THAT IS THE CORE OF THIS TYPE OF WOMAN.
Don't ask how i know. I have proof of them in both my dad's life, Mothers Life, and my Brothers life and my life.

YOU ARE CLOSING YOUR EYES. YOU ARE UNWILLING TO SEE AND ACCEPT THE TRUTH.

This is how the girls let it linger on and do the damage.
Securing themselves, living off your vulnerability, because they know you still care, you still want.
It's so easy to abuse.

RUN and don't ever look back again.
Start over.
And remember what you learned here.

God Speed. Be strong.
All is Well.

TLDR: She is using you.

To be honest it sounds as if she doesn’t really understand the severity of it and how it affects you. I would straight up tell her that you don’t like that she’s still friends with someone she obviously had/has those type of attractions to while you and her are together. It’s just not healthy for the relationship in any form

I agree with every one else. . .
What mark said was very very negative but sadly very very true about a lot of women(and men)
It's easier to stay in an unhappy relationship and cheat than it is to leave.
Even if you end up hating the other person,the moment you end it,you still feel full of regret and what ifs, even if you have a new person to jump straight to ,you still feel regret.

She clearly has no care for your feelings. If I was caught doing that with a Male friend ,I would feel ashamed to ever speak to him again whilst with the partner that caught me. . . .

I think you need to sit down and explain to her that you aren't happy about it. That it still is cheating and if it had been physical cheating, would she still be hanging out in person with the person she cheated with? If her answer is yes to that. Then she's selfish and you are just her ticket to an easy /secure life. If she says no, then point out that cheating is cheating, no matter how it's done, via text,on cam ,in person. . .it's still breaking the trust and to earn that trust back,the other person needs to be out of the picture

Maybe ask to swap phones as mark said,see if she's willing to.
Personally, I would say no to that to my partner because I often have private conversations with friends that they wouldn't want others to know about. But that is just me.
He knows my unlock code(says he doesn't but I've told him it enough times so he can send send  message for me when driving)so he knows my phone is "accessible" if need be.

Anyways . I hope you have the balls to put your foot down although the outcome may be very negative