Why not the following:
- use sipbroker PSTN like IVR
- ask for voxalot account number;
- accept PIN [could be the VM PIN] (perhaps the VM PIN will be too insecure...);
- ask for first number;
- ask for second number;
- ask for number of seconds delay [defaults to 15].
Then the caller simply hangs up and waits for the first leg of the call to call the phone he is at.
Use voxalot dial plans to route call to both numbers -- no reliance on dodgy or non-existant mobile signal.
There will be a short call to the closest PSTN gateway number to enter the required information from the initiating phone service.
Edit: added login to voxalot section and tidied up.