This is the screen used to compose and send an SMS message to a customer. For background, see Concepts|SMS.

The fields are as follows:
Phone - the phone number to which the SMS will be sent. If the customer has multiple phone contact numbers that are SMS enabled, then there will be a pull-down list for you to select from.
Message - the text of the SMS message. The maximum length depends on the maximum number of parts set for your SMS provider. The counter on the top of the box (138/2 in the above sample) shows how many more characters can be entered in the current part of the message and how many parts are being used. So 138/2 indicates that this will require a 2 part message and that 138 characters remain in the 2nd part. It you use a langauge with multi-byte characters (eg "東區動物醫院溫馨提示:Ngau Ngau的預約時間為今天10:30 AM 如需更改或取消預約請致電3456 7890 謝謝") then the counter will not be accurate. However, if the message gets too long, when you attempt to send it, you will get an error message like "Failed to validate: SMS is too long: 7 parts were entered but only 3 are supported"

Press the OK button to send the SMS.

Note that you can use macros when composing the message. You may find the following expression to be useful when building the macro called say 'smsr' using Administration|Lookups|Macros

 concat($patient.name,' is now due. Pls call ",party:getTelephone($location),' for an appointment. Thanks ',$practice.name) 

You could also create suitable macros for each vaccination, eg rg6 for 'Reminder: G6 Vaccination - "

Then you can simply type rg6 smsr into the SMS message text and press Enter to expand it to generate the text:
Reminder: G6 Vaccination - Muffet is now due. Pls call 9123 4567 for an appointment. Thanks Pets-R-Us

Note however, that the above macro will fail to expand if there is no current patient.

If you are using a macro that expands to a large amount of text - for example:

concat('Reminder: ',$patient.name,'.
Your appointment for ',$appointment.reason,' is on ',date:format($appointment.startTime,
"EEEE dd/MM/yy 'at' h:mma"),
'
Pls call ',party:getTelephone($location),' if you cannot attend. Thanks')

there is a possibility that the resulting text is longer than the maximum allowed. If this happens, then what you entered as the macro (eg @apr - for appointment reminder) will disappear, but the SMS text will be blank. If you press OK, then you will get a message like "Failed to validate: SMS is too long: 4 parts were entered but only 2 are supported".