When trying to add a Line URI or sip phone number to a response group.
Check for stale records in Active Directory
run command from Lync PowerShell Module
And check for messages in "Yellow"
WARNING: "RegistrarPool" with identity "594603069" assigned to
"sip:1905555555@sipdomain.com" has been removed from configuration store.
Below that showed the following
Identity : CN={5bf72950-f903-4177-8887-3635a433618f},CN=Application Contacts,CN=RTC Service,CN=Services,CN=Configuration,DC=Domain,DC=local
RegistrarPool : 594603069
HomeServer : CN=Lc Services\0ADEL:c5c61b5f-19a0-49a0-9aa3-e6841778f1d1,CN=Deleted Objects,CN=Configuration,DC=Domain,DC=local
OwnerUrn : urn:application:RGS
SipAddress :sip:1905555555@sipdomain.com
DisplayName : Company Name
DisplayNumber : +19055555555
LineURI :tel:+19055555555
PrimaryLanguage : 0
SecondaryLanguages : {}
EnterpriseVoiceEnabled : True
Enabled : True

Locate the Identity in Active Directory using ADSIEdit
And Delete the "CN" located at the end of the Identity String ie. "CN={5bf72950-f903-4177-8887-3635a433618f}"
And try and add your Line URI or Response Group SIP Address.