Hopefully you’ve completed the book having succeeded at implementing the areas you were interested in. Here are a few jumping-off points for further research and experimentation.
To connect your handsets with “real” phones in the outside world, you will need an Internet Telephony Service Provider (ITSP). ITSPs accept the SIP signaling and RTP media coming from your network and bridge them to the PSTN.
Finding ITSPs for voice service is straightforward. There are already online directories for these companies and a variety of plans are offered from prepaid minutes to flat monthly rates. Inbound phone numbers, or Direct Inward Dials (DIDs), are also offered in the majority of countries around the world. Many of these companies also supply instructions for how to connect to them when using Asterisk and can provide technical assistance for your installation.
SMS is more difficult. There are not very many ITSPs offering SIP MESSAGE to SMS service. Providers that do offer this service do not usually pair it with a voice DID so a single handset will have a different caller ID when making voice calls than when it sends SMS messages. This situation is improving but for now alternatives are needed.
Other providers offer Short Message Peer-to-Peer (SMPP) gateways or HTTP REST API gateways. Again, availability across all DIDs is unpredictable and some extra glue is needed to connect from SIP MESSAGE to either of these solutions. ...