The telephony designator "Sunny Air HTBD" consists of the telephony designator of the aircraft operating agency (Sunny AIR), followed by the last four characters of the registration marking of the aircraft (HTBD): Type b) => abbreviated form consists of the telephony designator of the aircraft operating agency, followed by at least the last two characters of the call sign "Sunny Air BD".

ICAO Annex 10
Volume II - Chapter 5

5.2.1.7.2 Radiotelephony call signs for aircraft
5.2.1.7.2.1 Full call signs
5.2.1.7.2.1.1 An aircraft radiotelephony call sign shall be one of the following types:

Type a) The characters corresponding to the registration marking of the aircraft; or
Type b) The telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft;

Type c) The telephony designator of the aircraft operating agency, followed by the flight identification.

Note 1: The name of the aircraft manufacturer or of the aircraft model may be used as radiotelephony prefix to the Type a) call sign.

Note 2: The telephony designators referred to in Types b) and c) are contained in Doc 8585 - Designators for Aircraft Operating Agencies, Aeronautical Authorities and Services

Note 3: Any of the foregoing call signs may be inserted in field 7 of the ICAO flight plan as the aircraft identification. Instructions on the completion of the flight plan form are contained in PANS- ATM, Doc 4444.

5.2.1.7.2.2 Abbreviated call signs

5.2.1.7.2.2.1 The aircraft radiotelephony call signs shown in 5.2.1.7.2.1.1, with the exception of Type c), may be abbreviated in the circumstances prescribed in 5.2.1.7.3.3.1. Abbreviated call signs shall be in the following form:

Type a) - the first character of the registration and at least the last two characters of the call sign;
Type b) - the telephony designator of the aircraft operating agency, followed by at least the last two characters of the call sign;
Type c) - no abbreviated form.

Note: Either the name of the aircraft manufacturer or of the aircraft model may be used in place of the first character in Type a).

Table 5-1 Examples of full call signs and abbreviated call signs
(see 5.2.1.7.2.1 and 5.2.1.7.2.2)