Resolve ambiguous names by using EWS

Question

I have a question about "Resolve ambiguous names by using EWS :"
I have a list of usernames "use-r1@Stuff .com, #user2@Stuff .com, 2user@Stuff .com" and I'm searching username by <keysearch>
but I encountered the following cases :

• when I input "r" the results return all usernames
• when I input "#" the result is "#user2@Stuff .com"
• when i input "-r" it returns an error is "ErrorNameResolutionNoResults"
• finally when I input number "2" it returns only "2user@Stuff .com" and without "#user2@Stuff .com"

this is my code :
<ResolveNames xmlns="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" ReturnFullContactData="true"> <UnresolvedEntry>keysearch</UnresolvedEntry> </ResolveNames>

So I have some questions :

• The searching will depend on which part of the username? the first character, or match with <keysearch>
• Is there any way to unify the search, such as searching based on the first character of the username?