iMessage says delivered but it isn’t…

This problem has been on and off for almost a year now between my phone and one other. We will send each other messages (either iMessage or WhatsApp) and it will say it has been delivered but the other person won’t receive it.. then typically a few more will be sent and after a period, usually a day or so, all the messages will suddenly come through together. Calls between our numbers often won’t connect either. I think the problem might be with the other persons phone as my messages seem to reach him more often than his reach me. The phone has been looked at in the shop several times and it is usually fixed for a short time then happens again, with no clear reason about why it keeps happening!

Has anyone else had this? I can’t find answers anywhere and it’s driving me mad!