Cell phone usage in Russia

Irina,
While I do suspect it's just another plug for some kind of service or website, I will take the time to respond -in broad brush strokes, if you don't mind :-)

Assuming you are from the US, you have two main options.

1. Get a reasonable international service plan from your provider or change the provider (TMobile and projectFi are arguably the two best options). With projectFi all calls made when not on WiFi will be 20 cents a minute, a gig of data is 10 dollars, all texts are free (voice mail is transcribed and sent to you as a text, too).
2. Get a local sim with decent data and use whatever messenger/video call service you choose (WhatsApp, Skype, Viber, Facebook msgr, telegram, etc). Also, use hotspot tethering to have your US phone always connected to WiFi.

I typically use both. Over the most recent two-week trip to Moscow my total phone-related expenditure was capped at about 15 dollars (9.40 for projectFi and about 5 for unlimited Beeline service)

I don't know what country you live in or what mobile service you have, so I have no idea how to answer your question. I had US T-Mobile service when I traveled to St. Petersburg in 2016, and their international roaming plan covered Russia, so I had free unlimited data and texting plus 20 cents/minute calls. I found my smart phone with Google Maps essential for navigating the city walking and using the bus system - saved me an enormous amount of hassle.

Thank you so much for your replies. So if we are on WiFi we can use FB without additional charges if we have an international plan, right? I guess I should call my provider.

Ahem...
if you're on WiFi you can use pretty much whatever you want app-wise, regardless of your plan. Calls and voicemail retreival over WiFi is free on projectfi for sure, but I have my doubts it would be free on TMobile (I've never used it). Just to clarify: project FI iş Google's own mobile network that only works with select few Android phones, so it might not be the way to go with your clear iPhone preferençe.

Bottom line - get a local sim from one of the big three (beeline, MTS, megafon). It should cost anywhere between 5 and 10 dollars, with that same amount preloaded. Should last you through a couple of weeks. Make sure your iphone is unlocked, though.

Just an update: unfortunately, projectFi does NOT support voice calls over WiFi in Russia. So any call will still be charged at 20 cents a minute, even if WiFi reception is strong.
Makes the case for going local SIM way even stronger.

You can still use Google Hangouts, Skype, and WhatsApp to make calls in Russia - for free if the other person also has the app, and a few cents/minute to phone numbers. WhatsApp is very popular in Europe. Hangouts also allows free calls to US phone numbers even landlines. I made some free calls with Hangouts while I was in St. Petersburg two years ago, when I had T-Mobile (I use Hangouts to avoid the 20 cents/minute cost calling direct with T-Mobile.)

Ditto the above information on tMobile. We were on a Baltic cruise last July and found they had excellent coverage in Europe (also good coverage on a Panama Canal cruise early this year and in the UK last week) with free wifi and texting and 20 cents per minute voice. We didn't use the voice much until we were at a location that that also had wifi and used voice through that service when needed. The tMobile coverage was very good (maybe better than they provide in the USA). While in St. Petersburg we used the Russian tour company, TJ Travel, and they provided wifi in their vans. We did put our phone in the airplane mode when we left each port so as not not pay roaming charges when we left tMobile coverage areas. The phone was handy for photos, but I also had my Nikon which did prove a little heavy and difficult in the crowded museums. It does take good photos though most people we saw used cell phones for picture taking (watch out for everyone taking selfies crowding the things you want to photograph!