Are special characters supported?

BudgetSMS supports both the standard GSM 03.38 character set, as any other character not listed in the standard GSM 03.38 characterset.

SMS messages sent with characters in the standard GSM characterset will have a length of 160 chars. when sending concatenated messages, the length of each SMS is calculated as 153 characters.

An SMS is sent as unicode when at least 1 of the characters in the message is not present in the standard GSM characterset. When a message is sent as unicode, the length of 1 SMS is 70 characters. When sending a concatenated unicode message the length per SMS part is 67 characters.

Calculation of messagelength standard GSM characterset

Below is a link added to Wikipedia with the GSM characterset. There are 2 tables. All characters in the left table count as 1, all characters in the right table count as 2.
Examples: "This is a test message" counts 22 characters, all from the left table. This means the SMS length is: 22
"This is [another] test" also counts 22 characters, but 2 of them ([ and ]) are from the right table: total SMS length: 24
(20 * 1) + (2 * 2) = 24

Other limitations might aply

please be aware that some destinations and/or routes have restrictions on what is possible regarding charactersets. For example: messages to Japan cannot be with Chinees or Arabic characters. They will not be supported by the operators in Japan.
Some (bulk)routes have restricted options for sending SMS, this can also include restrictions on the used charactersets.

See also: https://en.wikipedia.org/wiki/GSM_03.38#GSM_7-bit_default_alphabet_and_extension_table_of_3GPP_TS_23.038_.2F_GSM_03.38