A call from Person A to a contact of Person B. Viber Vulnerability?

Question

Viber

My mother and I live in different countries and communicate exclusively through Viber.
We do not use international telephone communication, only viber.

My phone number in my mother's home country/country of residence is known only to her and some work contacts from Telegram. There is no connection between my work contacts and my mother.

A few months ago, my mother ordered a brick for construction work from a certain Ruslan.
She called him from phone to phone (not via viber).
They didn't contact again.

Today I noticed a missed call in Viber from a certain Ruslan. He called 3 times.
After that, I received a conference call in viber from Ruslan and my mother.
I didn't answer.

A minute later, my mother called and said that she had received a conference call from Ruslan and me.
And before that, Ruslan called her in Viber.
She answered the phone, but there was silence on the other side, then the call was hung up.

Question: how could Ruslan have my phone number?
He never picked up my mom's phone.
Help, please, advice, I will be very grateful.

Answer 1

Most likely, your mother accidentally created a group of you, Ruslan and herself, with all the ensuing consequences.

Answer 2

you will not understand the advice, there are technical terms called "routing", "bugs", "test version"
therefore, according to the viber, the answer is - GRAPH is there? So the call can be
PS and yes, Ruslan didn’t have your number (well, if he didn’t save it now)