[Serusers] interdomain calls from numeric only UACs.

Hi All, How do people deal with calls between virtual domains on the same ser server from phones that can only easily dial numeric numbers and not proper SIP uris? Example, two users, 2000(a)sipdomain1.com calls 2000(a)sipdomain2.com ? They are different users, as the domain distinguishes them, but for a hard phone (like a analog phone connected to a ata, or a grandstream phone) the user can only really _easily_ dial a simple number? I'm sure this subject has been broached, it leads to the bigger question of the usefulness of sip uris in context of traditional telephones? I can think of effective ways around it by using prefixes and such, but I want to hear about other peoples hopefully more elegant approach :) Thanks, -Jev