Thanks for the reply.
It turns out that voice capability was disabled on the dongle. After enabling it, I’m able to make calls, but I only get one-way audio on inbound calls. If you have any suggestions for fixing that, I’d be happy to read them.
The SMS issue remains, but I think it’s because Linphone’s text messaging works only for SIP, not SMS. I probably need to use something else. For instance e-mail, XMPP or the web interface that chan_dongle offers.