this is exactly what I am doing , when I dial 9001 (custom app code), I am getting the error, even cc-16 is hard coded in my custom.conf file
My custom.conf file
[custom-contexts] (+)
exten => cc-16,1,Noop()
same => n,Gosub(email-notify-DM-ST,Sales-9003,1(T5_cos-all,9003,1))
[email-notify-DM]
exten=> _.,1,Set(user=admin)
same=>n,Set(Answer=${CURL(https://example.com/admin/client_data_received?client_id="${CALLERID(number)}"&token=adFki1234861252235fgh1))
same=>n,Goto(${ARG1},${ARG2},${ARG3})

LOGS:
-- Executing [s@sub-local-dialing:5] Return("PJSIP/T5_51001-0000006c", "") in new stack
-- Executing [s@T5_cos-all:12] GotoIf("PJSIP/T5_51001-0000006c", "0?end-call") in new stack
-- Executing [s@T5_cos-all:13] Set("PJSIP/T5_51001-0000006c", "OUTBOUND_PROFILE=") in new stack
-- Executing [s@T5_cos-all:14] GotoIf("PJSIP/T5_51001-0000006c", "1?post-dialing") in new stack
-- Goto (T5_cos-all,s,16)
-- Executing [s@T5_cos-all:16] Goto("PJSIP/T5_51001-0000006c", "T5_cos-all-post,9001,1") in new stack
-- Goto (T5_cos-all-post,9001,1)
-- Executing [9001@T5_cos-all-post:1] Gosub("PJSIP/T5_51001-0000006c", "sub-set-call-vars,app-incoming,1") in new stack
-- Executing [app-incoming@sub-set-call-vars:1] NoOp("PJSIP/T5_51001-0000006c", "Incoming Call App CDR") in new stack
-- Executing [app-incoming@sub-set-call-vars:2] GotoIf("PJSIP/T5_51001-0000006c", "0?:return") in new stack
-- Goto (sub-set-call-vars,app-incoming,6)
-- Executing [app-incoming@sub-set-call-vars:6] Return("PJSIP/T5_51001-0000006c", "") in new stack
-- Executing [9001@T5_cos-all-post:2] NoOp("PJSIP/T5_51001-0000006c", "Custom Application: Tenant CC Test") in new stack
-- Executing [9001@T5_cos-all-post:3] Goto("PJSIP/T5_51001-0000006c", "custom-contexts,cc-16,1") in new stack
**-- Goto (custom-contexts,cc-16,1)**
-- Executing [cc-16@custom-contexts:1] NoOp("PJSIP/T5_51001-0000006c", "custom-contexts") in new stack
-- Executing [cc-16@custom-contexts:2] Gosub("PJSIP/T5_51001-0000006c", "custom-contexts,s,1(ext-queues,9003,1)") in new stack
[2021-11-21 22:45:10] ERROR[20241][C-0000006d]: app_stack.c:593 gosub_exec: Attempt to reach a non-existent destination for Gosub: (Context:custom-contexts, Extension:s, Priority:1)