Only 1 filename by Monitor() on incoming PSTN calls

When Asterisk Monitor()'s incoming calls over PSTN trunk only one filename is created:
No timestamp in filename, which means I only have the most recent call, duh. From the dialplan:
exten => s,1,Answer()
exten => s,2,Monitor(wav,mb)
exten => s,3,Macro(stdexten,100,SIP/RS-O&SIP/RS-W); Dial SIP phones RS-O and RS-W. I am running Asterisk 1.0.9 with an X100P clone (if that’s a factor).

Strange: When monitoring [internal] calls, this timestamp is created, I get different filenames every [internal] call.

Is an update to 1.2.1 in order… or is there some other problem? Workaround needed? THX

try exten => s,2,Monitor(wav,${CHANNEL}-${DATETIME},mb)

Thank you IronHelix,
After a short break… picked back up on the issue.

Works! Funky side effect: ${CHANNEL} has value Zap/1-1 so log files are stored in a newly created directory /Zap
Not exactly neat.
I ended up limiting filename to ${DATETIME}, which is sufficient to match the file to the information given in