Hi
Snoms are likely trying to use rtp encryption. Turn it off in identity->rtp on the Snom.
You can use dynamic HTTP provisioning for Snom and Yealink on V4 (and later releases of V3). Turn it on in Globals->Services (PnP provisioning=YES). You can have multiple servers on the same subnet all using PnP provisioning. They will only respond with a provisioning stream to phones that have been specifically defined to each server so they won't conflict with each other and they won't conflict with phones getting their stuff from TFTP.
No idea how you would do it in DHCP Options so I guess you'll need to do some research if you want to do it that way. In general we are moving towards HTTP provisioning for all phone types we support (Snom, Yealink, Aastra, Polycom, Cisco). Aastra and Polycom will be added in a near release, Cisco probably a little later. In these releases, TFTP server will be turned off by default but you will still be able to turn it on, at least for now.
Kind Regards
S