Thanks. I'm trying to work around the pathetic mobile signal where I live. With the hint about syncing, I got the two of them synced (had neglected that) and have been able to use wifi to phone my landline with the iPhone and the reverse also, and to get my voicemail on the other mobile (which is the one I use for SMS messages and ordinary calls).
Then the real problem, which is attempting to work around the bad mobile signal to get SMS messages to and from that other phone. May need an app or something.
Oh, this is Vodafone in Ireland - the other networks seem to be just as bad here where I live. (Beautiful but in the hills!)