I tried that and got this from the console. I did remove the default MOH wav files to force the mp3 file
saint*CLI>
-- Executing [400@internal:1] AGI("SIP/401-099e4a60", "selintra|OutCluster|400") in new stack
-- Launched AGI Script /var/lib/asterisk/agi-bin/selintra
-- AGI Script Executing Application: (Set) Options: (CDR(accountcode)=callerCluster)
-- AGI Script selintra completed, returning 0
-- Executing [400@qrxvtmny:1] AGI("SIP/401-099e4a60", "selintra|InCall|") in new stack
-- Launched AGI Script /var/lib/asterisk/agi-bin/selintra
-- AGI Script Executing Application: (Dial) Options: (SIP/400|45|tT)
-- Called 400
-- SIP/400-09a0e240 is ringing
-- SIP/400-09a0e240 answered SIP/401-099e4a60
[Aug 31 08:53:22] WARNING[10617]: res_musiconhold.c:660 get_mohbyname: Music on Hold class 'default' not found
[Aug 31 08:53:22] WARNING[10617]: res_musiconhold.c:660 get_mohbyname: Music on Hold class 'default' not found
-- AGI Script selintra completed, returning 0
== Auto fallthrough, channel 'SIP/401-099e4a60' status is 'ANSWER'
-- Executing [h@qrxvtmny:1] Hangup("SIP/401-099e4a60", "") in new stack
== Spawn extension (qrxvtmny, h, 1) exited non-zero on 'SIP/401-099e4a60'
saint*CLI>