ADTRAN used to have a NetVanta VPN client available at one point through SafeNet, however ADTRAN's contract with Safenet has since ended and that particular VPN client has since been discontinued. Also, it did not provide support for Windows Vista and 7 users.
While ADTRAN does not specifically certify or support other VPN clients, all AOS VPN capable products are compatible with any IPSEC VPN client that implements IKE and supports Mode-Config.
ADTRAN has done some internal testing with ShrewSoft and developed a configuration guide for users to set up their VPNs with. This, coupled with the fact that it is free to download and supports Windows Vista and 7, usually makes it the most convenient option for AOS users at this time.
Besides Shrew Soft, we have had users have success using clients like the NCP Greenbow client and Safenet's SoftRemote client.
ADTRAN is also looking into introducing a new officially supported VPN client here in 2012.
I hope this answers your questions. Please feel free to reply to the thread if you think of any further questions regarding this.
SuperNoor! Great to hear from you.
Thanks for helping me understand the state of things with mobile clients. I really like the Shrew Soft client, and our customers who use it do pretty well with it. Sometimes we just feel a little hokey saying, "Just download this third party client called Shrew Soft," especially when competing with Cisco or SonicWALL vendors who have nicely-branded, officially supported clients. Even simply having a paid ADTRAN client available would be great to present to larger, corporate customers, and Shrew's free client would remain an option for smaller offices on a tight budget. Looking forward to seeing that.