Unless, of course, you have a regular time to ring each other each week - which is what happens in our case. Whoever rings who, the call is expected.
On those rare occasions that a phone call does come at an inconvenient time, you just apologise, agree a convenient time, and ring back.
There is nothing to beat the freeflow of a conversation that wanders here there and everywhere, like a casual cnversation over coffee. Actually DD and I usually settle down with a cup of tea each while we enjoy each others company.