I just got a phone call from +8001836404. I answered because, in the few seconds before picking up I misread the number. It turned out to be someone claiming to be from Virgin Media, trying to sell me Sky. After a few minutes I gave up on them but they got 8 minutes of my time.
If you haven't handed over any sensitive information you are fine.
There is no charge to receive calls (unless you are roaming of course). I had several calls from a number like that last month offering some sort of value Virgin Mobile contract (I don't believe anything said in a sales call and they don't want to send a letter, so that went well) with no problems.
How much it would cost to actually call such a number I don't know...