It all depends on what is in the existing router and what you are trying to do. If you are adding FXO, FXS, and/or T1/E1 cards but you are not using CUBE functionality or SRST functionality, you need is the UC feature license, DSPs, the cards, and possibly a DRAM and flash upgrade.
For your reference, I have provided an example "stock" build of a 2921/K9 and 2921-V/K9. By stock, I mean no options were added other than smartnet and a power cable. The 2921 comes with some CUBE licensing, IOS UC licensing, a PVDM3-32, and more memory. If you are buying a new gateway, it is usually cheaper to buy this package rather than buying the 2921/K9 and adding the features.
| 1.0 | CISCO2921/K9 | Cisco 2921 w/3 GE 4 EHWIC 3 DSP 1 SM 256MB CF 512MB DRAM IPB | N/A | 14 days | No | 1 |
| 1.0.1 | CON-SNT-2921 | SMARTNET 8X5XNBD Cisco 2921 | 12 month(s) | N/A | No | 1 |
| 1.1 | SL-29-IPB-K9 | IP Base License for Cisco 2901-2951 | N/A | 14 days | Yes | 1 |
| 1.2 | HWIC-BLANK | Blank faceplate for HWIC slot on Cisco ISR | N/A | 21 days | Yes | 4 |
| 1.3 | ISR-CCP-EXP | Cisco Config Pro Express on Router Flash | N/A | 21 days | Yes | 1 |
| 1.4 | MEM-2900-512MB-DEF | 512MB DRAM for Cisco 2901-2921 ISR (Default) | N/A | 14 days | Yes | 1 |
| 1.5 | MEM-CF-256MB | 256MB Compact Flash for Cisco 1900 2900 3900 ISR | N/A | 21 days | Yes | 1 |
| 1.6 | SM-D-BLANK | Blank faceplate for DW slot on Cisco 2951 and 3925 | N/A | 21 days | Yes | 1 |
| 1.7 | SM-S-BLANK | Removable faceplate for SM slot on Cisco 290039004400 ISR | N/A | 21 days | Yes | 1 |
| 1.8 | S29UK9-15401T | Cisco 2901-2921 IOS UNIVERSAL | N/A | 14 days | No | 1 |
| 1.9 | PWR-2921-51-AC | Cisco 2921/2951 AC Power Supply | N/A | 14 days | Yes | 1 |
| 1.10 | CAB-AC | AC Power Cord (North America) C13 NEMA 5-15P 2.1m | N/A | 14 days | No | 1 |
| 1.11 | PI-MSE-PRMO-INSRT | Insert Packout - PI-MSE | N/A | 14 days | Yes | 1 |
| 2.0 | CISCO2921-V/K9 | Cisco 2921 Voice Bundle PVDM3-32 UC License PAK FL-CUBE10 | N/A | 14 days | No | 1 |
| 2.0.1 | CON-SNT-2921V | SMARTNET 8X5XNBD Cisco 2921 Voice Bundle | 12 month(s) | N/A | No | 1 |
| 2.1 | S29UK9-15204M | Cisco 2901-2921 IOS UNIVERSAL | N/A | 14 days | Yes | 1 |
| 2.2 | PWR-2921-51-AC | Cisco 2921/2951 AC Power Supply | N/A | 14 days | Yes | 1 |
| 2.3 | CAB-AC | AC Power Cord (North America) C13 NEMA 5-15P 2.1m | N/A | 14 days | No | 1 |
| 2.4 | FL-CUBEE-5 | Unified Border Element Enterprise License - 5 sessions | N/A | 21 days | Yes | 2 |
| 2.5 | PI-MSE-PRMO-INSRT | Insert Packout - PI-MSE | N/A | 14 days | Yes | 1 |
| 2.6 | SL-29-IPB-K9 | IP Base License for Cisco 2901-2951 | N/A | 14 days | Yes | 1 |
| 2.7 | SL-29-UC-K9 | Unified Communication License for Cisco 2901-2951 | N/A | 14 days | Yes | 1 |
| 2.8 | HWIC-BLANK | Blank faceplate for HWIC slot on Cisco ISR | N/A | 21 days | Yes | 4 |
| 2.9 | ISR-CCP-EXP | Cisco Config Pro Express on Router Flash | N/A | 21 days | Yes | 1 |
| 2.10 | MEM-2900-512MB-DEF | 512MB DRAM for Cisco 2901-2921 ISR (Default) | N/A | 14 days | Yes | 1 |
| 2.11 | MEM-CF-256MB | 256MB Compact Flash for Cisco 1900 2900 3900 ISR | N/A | 21 days | Yes | 1 |
| 2.12 | PVDM3-32 | 32-channel high-density voice and video DSP module | N/A | 21 days | Yes | 1 |
| 2.13 | SM-D-BLANK | Blank faceplate for DW slot on Cisco 2951 and 3925 | N/A | 21 days | Yes | 1 |
| 2.14 | SM-S-BLANK | Removable faceplate for SM slot on Cisco 290039004400 ISR | N/A | 21 days | Yes | 1 |
