This sounds like a good deal...the call filtering plus the MP3 ability for calls.
Hmmm...might be my first Treo software purchase.
Website Link:
http://www.velocityware.com/callfilter/cfinfo.htm
Info for those that wanna know:
CALLFILTER V1.3 FOR TREO 600/650
Manage your incoming and outgoing calls and enhance the capabilities of your Treo 600/650 dramatically. With CallFilter for Treo 600/650, you have the ability to block unwanted callers, automatically hangup on someone bothering you, and more! Stop telemarketers, annoying people, or anyone you just dont feel like speaking to at the moment from disturbing you. In addition, you can also take control of your incoming SMS Messages. CallFilter gives you complete control of incoming and outgoing communications. With the Call Completion, you can take quick notes about your last phone call and quickly schedule followup calls or meetings. For those calls you want to accept, you can use Midi, MP3*, WAV*, and WMA* files for your ringtone. In addition, you can enjoy unlimited Picture Caller ID, and innovate new informational ring screens!
FEATURE SUMMARY
<->Filter incoming calls based on phone number, name, category, blocked/no callerid, or company
<->Send unwanted calls to voicemail immediately or after a few rings, pickup and hang up on caller, ignore caller, or automatically answer *Enhanced*
<->Ability to play MP3, Ogg, Wav (requires Pocket Tunes), and Midi files for the Ring Tones *Enhanced*
<->Adds unlimited Picture Caller ID. You can even assign Pictures to Contact Categories, Companies, or Blocked Callers!*New*
<->Filter SMS messages. Filter not only by phone number, but can also filter based on the email address that sent the SMS message. *New*
<->Take action based on an SMS message. CallFilter can automatically startup an application, auto reply, delete the message, or suppress notification. *New*
<->Ability to actually SPEAK the phone number of the incoming call *New*
<->Ring Screen called EXTENDED INFO - Automatically displays contact info and notes about the incoming caller, so you can refresh your memory before you even answer the phone! *New*
<->Call Completion - Automatically display a form after a phone call, so you can quickly take notes, schedule follow up calls, or meetings with the caller. All notes and appointments are integrated with the Treo 600/650 Calendar, Memo Pad, Daily Log, and Contact Notes *New*
<->Ability to stop Space bar and other keys from accidentally answering your phone
<->Support for Multiple Cards (SD card and RAM Disk at same time)
<->Ability to Automatically Forward Calls when Charging
<->It detects when you have voicemail, and makes your green led blink quickly at 4 times a second, or set the LED color to solid Red, Orange, or Green *Enhanced*
<->Allows the Jabra Freespeak BT200 Bluetooth headset to be used with the Treo 600 *New*
<->Support Multiple Rulesets based on your current profile! Integrate seamlessly with third party Profile Managers such as Simple Profiles Manager, Profiler, or Profeo 600 Lite V0.8.7. When you switch profiles using your Favorite Profile manager, CallFilter automatically switches to the ruleset that corresponds to that Profile! *New*
<->Integrates with the Treo 600/650 Alerts Manager, so you can quickly see a summary of Calls and SMS messages that have been filtered *New*
<->Advanced Logging Screen allows you to zoom in on a log message to see extended details on what happened. *New*
<->Ability to play an background sound or annoucement when the phone is answered *New*
<->Ability to automatically return to the built-in phone app or TAKEphONE after entering Call Completion Notes *New*
<->Ability to assign a nickname to a individual, phone number, category, or company *New*
NEW FEATURES IN VERSION 1.3 
Added Treo 650 compatibility

Ring Screen Called LOCATION INFOAbility to Display Approximate Location of Incoming Calls

Ability to Filter Outgoing Phone Calls - Now you can restrict to whom calls are made

Includes built-in VelocityPlayer to allow MP3 and Wav Ringtones without external software (650 only)

Ability to Change the Color of the LED based on who is calling

Improved Performance and Speed, and file handling capabilities

Includes ability to require a password to see your rules, and a method to hide your rules