[route-call]
exten => 500,1,Set(PEERCODECS=${SIP_CODEC()})
exten => 500,2,NoOp(${PEERCODECS})
exten => 500,3,Set(PREFCODEC=${CUT(PEERCODECS|,|1)})
exten => 500,4,NoOp(${PREFCODEC})
exten => 500,5,Set(PEER=${IF($[ ${PREFCODEC} = ulaw]?bvox-ulaw:bvox)})
exten => 500,6,NoOp(${PEER})
exten => 500,7,Hangup()

CLI output
    -- Executing Set("SIP/2000-a37c", "PEERCODECS=gsm,ulaw,g729") in new stack
    -- Executing NoOp("SIP/2000-a37c", "gsm,ulaw,g729") in new stack
May 11 18:40:39 ERROR[17493]: app_cut.c:397 acf_cut_exec: Usage: Splits a variable's contents using the specified delimiter
    -- Executing Set("SIP/2000-a37c", "PREFCODEC=") in new stack
    -- Executing NoOp("SIP/2000-a37c", "") in new stack
May 11 18:40:39 WARNING[17493]: ast_expr2.fl:183 ast_yyerror: ast_yyerror(): syntax error: syntax error, unexpected TOK_EQ, expecting TOK_MINUS or TOK_COMPL or TOK_LP or TOKEN; Input:
  = ulaw