I do not think that CallManager can do provide dialtone - you may be able to get a H.323 gateway to do so but be careful about toll fraud.
The option I would look at is forwarding the callers to an Autoattendant IVR that prompts callers to enter the number they wish to reach.
Most falvours of Unity include Autoattendant functionality. If you do not have Unity then using a TCL script on a H.323 gateway may be an option.
The link below may help with the TCL script option.
http://angryciscoguy.com/jello/?p=45
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Edit
In my experience (mainly Quescom) GSM gateways are not great at dealing with DTMF tones from inbound callers - I would be interested to know how the Topex boxes work with this.