I recently came across Twilio.com that provides API facilities to implement voice or text capabilities through the internet. They aren't the first to provide this kind of service but the capability of the API and their pricing seem to make them worth looking at for anyone who has a need for this type of facility. The service is mainly intended for North America at this point however, they do support international outbound calls.
Twilio can get you your own phone number for $1 / month ($2 toll free) if you need it. It is pay-as-you go at 1 cent / minute for incoming voice, 2 cents / minute for outgoing voice or for SMS in either direction. They provide API facilities to put in a virtual PBX and other kinds of voice apps. They have a sample phone system app that can be used and examples of using the API in a number of languages, including PHP.
We see questions on a regular basis about implementing an SMS interface. For those who need incoming SMS (to their app) this seems like a pretty good option. You can do your own outbound SMS interface through email for free if you know the carrier for each number. This sort of interface lets the supplier (Twilio) sort out the carrier. For business use, you can also implement a pretty sophisticated voice app.
Their prices seem pretty reasonable. I don't have any personal interest (financial or otherwise) in this service, it just seems like something that may be useful for some developers and their customers.