Is there any way to add a PPTP "phone number" to the connection? This is required to work with my Thomson TG585 modem (http://forums.modem-help.co.uk/viewtopic.php?p=9347)...

This is actually documented in RFC2637, under 2.7. Outgoing-Call-Request, but I can't seem to add on in routeros

pptp is tunnel created over IP network.

Maybe you meant to create ppp interface that can be used to actually call-in somewhere or use to connect via 3G modem? If so, please check appropriate section on wiki.mikrotik.com

Are you serious? I know what PPTP is, and how it works, and what I want to achieve with it. I even referenced an RFC describing how the "phone number" is used in PPTP

My guess is that you are configuring a pptp client and want the connect-to parameter. Perhaps the wiki at http://wiki.mikrotik.com/wiki/Manual:Interface/PPTP would be of help.
If this is not the case, might I suggest a more detailed description of your application and problem?

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with 'Outgoing-Call-Request' one can force PPTP Server to establish some external connection (dial some number) when client connects

hydroksyde, can it be done in Windows client? I'd like to check whether it's used somewhere in real life

Are you serious? I know what PPTP is, and how it works, and what I want to achieve with it. I even referenced an RFC describing how the "phone number" is used in PPTP

Are YOU serious? You've got to be kidding... someone offers help and that is your response?