This is UNTESTED but I think it may work…
Let’s say you have written a custom dialplan in extensions_custom.conf to mung the Caller-ID before sending it to your cell phone. If you only need this to work when it goes to YOUR cell phone (in other words this is a one-shot thing for a specific destination), you can use the Custom Destinations module (under Tools) to create a link to your custom dialplan - in the Custom Destination textbox you’d use something like custom-my-dialplan,${EXTEN},1 where custom-my-dialplan is the name of the dialplan fragment you’ve put in extensions_custom.conf.
Then you create a Misc Application to assign a number to that Custom Destination (you can make this something no one would ever dial, such as 0000123, just so long as it doesn’t conflict with anything else in your dialplan. Or, you could use something more in line with your existing numbering plan and make it a speed dial number for your cell, for use from internal extensions only - it’s up to you). Now you have a number you can put in the follow-me (instead of your cell phone number).
The last thing to do (well, actually, probably the first thing, but it makes more sense to explain it last) is write the code to mung the outgoing Caller ID before sending the call to the outbound-allroutes context. You will probably get some ideas of how this might be done by looking at the sample dialplan fragments here:
http://www.freepbx.org/support/documentation/howtos/how-to-change-incoming-callerid
The only difference is that you are changing the outgoing caller ID, not the incoming. So, instead of sending the call to from-trunk,${EXTEN},1 (in the final Goto) you’d send it to something like outbound-allroutes,yourcellnumber,1 (replace yourcellnumber with the actual number of your cell phone).
In other words, without testing it, your dialplan in extensions_custom.conf might look like this:
[custom-my-dialplan]
exten => _X.,1,Set(CALLERID(num)=1xxxxxxx${CALLERID(num)}) ;(substitute the xxxxxxx part from your phone number)
exten => _X.,n,Goto(outbound-allroutes,1yyyyyyyyyy,1 ;(replace the yyyyyyyyyy with your cell number)
(Omit the 1 before the yyyyyyyyyy if your system supports 10 digit dialing)
This is just off the top of my head, I have not actually tested any of this. Also the limitation is that you have to create a separate custom dialplan fragment (and the pointers to it) for each cell phone you want to send a call to. Let us know if this actually works.