My friend is unable to see our summon signs nor are we able to see his. We even got as far as getting the name-engraved ring and choosing a god to have a better chance of syncing up together.
The only time he successfully summoned us was when he logged back in and was close to where we had our signs down. Otherwise no luck.
Cross-region play is enabled for all of us.
Because you mentioned successfully being summoned into his game but not that you were ever able to summon him into yours, I think this might be the issue. Have you made sure that your soul memories are in compatible tiers? A full breakdown can be seen here.
If you are attempting to summon with the White Sign Soapstone while using the name-engraved ring, you should be able to summon "Down 6, Up 4" tiers.
If you are 5-6 tiers below him, that would explain why you are unable to see his signs, yet he was able to summon you.
