Hello,
You should tell us more about your configuration. Why do you have ipvoice IOS? Are you using any voice features? I'm asking because if you are using voice features, the new IOS also should support them.
Second, you didn't mentioned if you are need remote or server ezvpn.
Anyway, if we can skip over the above setting, Cisco software advisory recommend the following for ezvpn with 2651:
c2600-adventerprisek9-mz.12.4-25c
minim flash 32 memory 128
This is just an idea, you can use it, but also depending on your requirements, the advanced enterprise IOS version may be too much, so you can stick with the same version, but different features set.
Also be aware that not all ezvpn features are supported in c2600 IOS.
Cheers,
Calin