SMS (or Short Message Service) is a means of communicating short text messages with mobile phones. Messages can be sent between mobile users' phones or between applications (e.g. voicemail systems, retail websites, banking systems) and mobile users' phones.
SMS in use on GSM mobile networks supports up to 160 characters when using the GSM default alphabet or 70 UCS2 characters (e.g. for Arabic and other non-Latin languages). When a message is longer than these limits, then a "long SMS" (or concatenated SMS) can be sent that uses segmentation and reassembly (SAR) to split the long message into multiple SMS message parts that use meta data to associate the parts. SMS can also carry non-text message payloads up to 140 x 8-bits in a single message.
When sending an SMS from one mobile to another, the sending mobile passes the SMS to their own mobile network's Short Message Service Center (SMSC), and this stage is known as the Mobile Originated (MO) SMS. The SMSC then attempts delivery of the message to the destination mobile, which may be on the same network or another interconnected mobile network. This latter stage is known as Mobile Terminated (MT) SMS.
SMS is defined in the 3GPP Technical realization of the Short Message Service (SMS) standard (23.040).
For information on concatenated SMS (aka long SMS), there's a good resource on Wikipedia: Concatenated SMS.
There are a number of free services available online for use in SMS testing. These cover SMSC simulators, protocol conformance, application stress testing and other areas. Some are listed below:
Specifications and services directly related to the SMPP protocol, the SMS-specific protocol for enabling SMS in applications.
SMS text marketing (#martech). Virtual Mobile Numbers. Virtual SIM Hosting. SIM Hosting.
The following numbering from Ofcom is accurate as at Thursday, 22 April 2021.