Weird how everyone has their own way and each way is "right" for them.
Generally I call first but only cause I usually gotta travel 'bout 100 miles before going round. Can be a little off putting if no-ones home
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As for my local mates, we usually meet up at bars and such so its always done by mobile.
My front door is on a buzzer, so if I'm busy I don't answer. I share the flat though so sometimes its not for me and I have to answer. With a buzzer system, even if I answer it don't mean I'm gonna open it...
As for your mate, the most sensible idea so far is tell his wife, ask her to call you when he's left to come over. If you are good friends (20yrs is a long time), this should be sortable.