I think the phone thing has been a miscommunication, by saying his friend ignored his messages most posters thought he must be attempting to organise his own social life. These comments were based on this, not attacks.
Things you can try: a play date with multiple children, or an email/WhatsApp/FB message to the whole class suggesting a picnic or "I saw x was on, shall we try and get a group together?" When there, talk about next week, " we wanted to see the lion king too, how about Tuesday?" Nail things down, take the lead. In my DCs class, one parent posted about a great deal at the trampoline place, lots of class bought pass and there are a couple of "I'm going today at 5pm," style messages each week. Approx 5 kids have been every time we've been.
If your son is shy and play dates are hard going for both kids, take a little control, set them up painting or a treasure hunt or something. Free play will come later, but he needs to have fun/be less anxious first. My shy DD is a little older and only starting to get it now, it's been hard work but it's paid off.
If you have young engineers or an Apple shop near you, you could try robotics camp. Look at set activities at museums and libraries. If you see the same child at two things, then you can chat to them/the parent (obvs this is an unwritten rule I've observed!)
Just getting out each day will really help, it gives him more conversation when he does make friends and pushes him out his comfort zone a little. Good luck!!