Unabale to attach a contact to multiple customers

For a contact to access multiple customer accounts through iReceivables, it has to be assigned to all those various customers with 'self service user' role.In order to do it, one has to define it for one customer and then select the same for other customers. I am unable to do it. The contact last name shows drop down list but does not show contact number and hence unable to assign the proper contact (since there are multiple contacts with same name).
Can any one help me? What am I missing?

Thanks.

Anil