Well if you already have the OGM programmed to take credit card payments, then you should be good to go and ready for testing within the hour. IVM can hook up directly with your VoIP account and start receiving calls as soon as you need them.
If you don't have the OGM programmed already.. this is not going to be an easy task, and even if you had a large team of programmers.. I would not recommend you try to develop a credit card payment system, test it, and then deploy it in just a couple of days. This goes for any major system, but IVM is simple enough that if you have the OGM ready it'll only take a few minutes to get setup.
The OGM (if it isn't programmed) will require that you ask customers for a credit card, take the input, store it to a variable which then executes an external program/plugin (that you will need to develop) that checks the credit card input to see if it is a valid number. Then you should either have the program interface with a credit card payment system directly (if you have one), so customers are charged automatically, or more likely store the credit card data in a database (using the database interface plugin) so someone can manually check it later and then charge the credit cards as needed).
The phones that can access your IVM system is entirely dependent on your VoIP provider. Most providers (assuming you have purchased an inbound phone number (DID) or a calling plan that includes an inbound number then anyone who calls the number whether it be from a land-line, mobile phone, or VoIP phone shouldn't have a problem using the IVR menu and OGM you've setup in IVM. Note that only touch-tone enabled phones (i.e. not pulse or rotary dialing phones) will be able to input digits into the phone system (as with any IVR system out there).
VoIP service providers usually will have your service setup instantly, although it is always a good idea to check their FAQ or similar just to make sure this is the case if time is such an importance.
A service that I use and find works well is Voice Network ( http://www.voicenetwork.ca ). They provide toll-free 1-800 numbers to anyone in the US/Canada for $1 month plus 4c/min (this is one of the best deals I've seen). They also offer other inbound numbers across the US (see http://www.voicenetwork.ca/usa_did.html ) - if you pick the pay-as-you-go plan, you can have up to 6 simultaneous calls, otherwise you will be limited to only two (on the flat rate / no per minute charge plan).
