Hi,
I would suggest to go with atleast 3825 router.
The Cisco 3825 Integrated Services Router provides the following support:
* Wire-speed performance for concurrent services such as security and voice , and advanced services at up to half T3/E3 rates
* Enhanced investment protection through increased performance and modularity
* Enhanced investment protection through increased modularity
* Increased density through High-Speed WAN Interface Card Slots (four)
* Enhanced Network Module Slot
* Support for over 90 existing and new modules
* Support for majority of existing AIMs, NMs, WICs, VWICs, and VICs
* Integrated GE ports with copper and fiber support
* Analog and digital voice call support
* Optional voice mail support
* Optional support for Cisco CallManager Express for local call processing in stand alone business for up to 168 IP Phones
* Optional support for Survivable Remote Site Telephony support for local call processing in small enterprise branch offices for up to 336 IP phones
For the interfaces and modules avaialble on 3800 router, Please find the link below :
http://www.cisco.com/en/US/products/ps5855/products_relevant_interfaces_and_modules.html
HTH, Please rate if it does.
-amit singh