What kind of a statistical bias would explain having 1 million Belariusian phone numbers claiming having voted for a candidate, that officially received around 0.5 million votes total?

Adulthood necessary to vote could be one?

You’re ignoring the finite population and the actual numbers involved.

It is simultaneously possible for the results to have statistical issues while also being good enough to provide utility as evidence for drawing some conclusions. You can both be right.

Indeed

What does that have to do with the fact that the number of telegram users that voted for the opposition candidate exceeds the official number of voters that voted for the opposition candidate?

Phone number is not an unique identifier. There are plenty of people who use multiple phone numbers.

While I don't believe in Belarus official results, that Telegram evidence is not a real evidence. It's just a hint.

I very highly doubt around double the amount of ”official” voters would have a second belarussian number.

Also, they have to decide to actually use that second number to cheat in a Telegram poll significantly more often than people of the other party.

Otherwise you can only draw your error bars equally in both ways at once, and then they need to be pretty large before the numbers stop saying what they clearly are saying.