Função PL/SQL com erro

Olá,

ao chamar a função, estou a receber o erro:

"ORA-01422: extracção exacta devolve mais que o número pedido de linhas"

create or replace FUNCTION "CALC_ADDT_SIM_DISC" (ACCT_INPUT VARCHAR2)

RETURN VARCHAR2 IS

SIM_MSISDN VARCHAR2(20);
BEGIN

 SELECT CASE

 WHEN ADDT_DISC_1 = 1 THEN mu.MSISDN

 WHEN ADDT_DISC_1 > 1 THEN (SELECT DISTINCT t02.MSISDN

 FROM CDM.DMKV_TRANSP_VAR_RESULT_02 t02

 WHERE SERV_ACCT_OLDER_MSISDN_FLG = 'Y' AND
 PRICING_PLAN_TYPE IN ('valor1', 'valor2')
 AND t02.DW_SERV_ST_ID='100000003' AND t02.CUST_ACCT_EXT_KEY = ACCT_INPUT

 )

 END MSISDN_ADDT_DISC

 INTO SIM_MSISDN

FROM(

 SELECT DISTINCT CUST_ACCT_EXT_KEY, COUNT(CUST_ACCT_EXT_KEY) AS ADDT_DISC_1
 FROM MOBILE_UPSELL
 WHERE FLAG_ADDITIONAL_SIM_DISCOUNT = '0'
 GROUP BY CUST_ACCT_EXT_KEY

 )t,
 MOBILE_UPSELL mu

WHERE t.CUST_ACCT_EXT_KEY = mu.CUST_ACCT_EXT_KEY AND t.CUST_ACCT_EXT_KEY = ACCT_INPUT;

RETURN SIM_MSISDN;

END;

	O que estou a fazer de errado?

	 

	Obrigado