Snom M100/M10 SC cannot rejoin shared call

I dialled an outgoing call on one M10 handset and someone else joined the call on another M10 handset on the same base station by pressing the line button (L1). After a while I disconnected from the call and the other handset continued the call. Later on, the call was still going, and I tried to rejoin by pressing the L1 button, but after "Connecting..." I just get the message, "Unable to call. Try again". Should this pattern of usage be expected to work? Is this a known bug? I can't find anything about it.