[Serusers] Extension alias for sip url??

Hi, I am wondering what is the usual position with translating between extension numbers and sip urls.... I mean I have a grandstream bt100 sip hardphone registered with ser with an ext no 2001 and I also have a windows messenger sip client with sip url aisling@domain_name. The windows messenger client can ring the hardphone by dialing 2001@domain_name.....But is there a way that the hardphone can ring the sip url....The hardphone only supprts extension numbers and ip addresses..... Is there a way of using an extension alias to ring the ip url or something...Has anyone else had this problem or what do people normally do?? Thanks very much, Aisling -------------------Legal Disclaimer--------------------------------------- The above electronic mail transmission is confidential and intended only for the person to whom it is addressed. Its contents may be protected by legal and/or professional privilege. Should it be received by you in error please contact the sender at the above quoted email address. Any unauthorised form of reproduction of this message is strictly prohibited. The Institute does not guarantee the security of any information electronically transmitted and is not liable if the information contained in this communication is not a proper and complete record of the message as transmitted by the sender nor for any delay in its receipt.