EnvayaSMS typically only needs to be installed on one Android phone.
Suppose that you have installed EnvayaSMS on an Android phone in Dar es Salaam,
Tanzania, with phone number
+255987654321 (local number
with GSM service from Vodacom.
Suppose also that your web server is
example.com, and its servers are located in a datacenter in London.
In Dar es Salaam, you would put the Android phone in an office with internet access and a Wi-Fi router, and connect the phone to the Wi-Fi network. (Wi-Fi is not strictly necessary, since EnvayaSMS will also work over the phone's mobile data connection, but Wi-Fi is likely to be faster, cheaper, and more reliable.)
Then your network would look like this:
Typically your organization will need only one phone running EnvayaSMS in a particular country. However, it is possible to use multiple phones to load-balance requests for higher throughput, to offer different user services, or to serve different mobile networks.
Here is an example of how EnvayaSMS would forward an incoming SMS to your server, and send a SMS back as a reply:
0555551234, sends SMS message to
Since users generally are charged only for outgoing messages in Tanzania, the user would be charged for sending one local SMS message, and your phone would also be charged for sending one local SMS message.
EnvayaSMS uses a different process to send SMS notifications that were not in response to an incoming message.
Assume now that
example.com has multiple phones using EnvayaSMS in different countries,
and it is configured to send all messages to phone numbers with a
255 prefix (Tanzania's country code)
via the cell phone with number
example.comwants to send an SMS message to
example.comidentifies that the recipient number has a
255prefix, and chooses sender phone number
example.comfor outgoing messages for its phone number
+255987654321, the server responds with the message to send
example.comvia HTTP that message was sent
example.comremoves the SMS message from its queue of outgoing messages.
In these examples, even though SMS messages are being communicated between Dar es Salaam and London, all SMS messages are sent domestically and are not charged as international SMS messages.