Or a cleaner way is to do something like this in extensions.conf.
Or wherever your repeater extension is defined.
[radio-gui]
exten => 28260,1,Playback(/path/to/file.ulaw)
exten => 28260,n,Wait(3)
exten => 28260,n,rpt,28260|P
>From the iaxRpt access definition on one of my nodes.
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