[OpenSER-Users] Problem with otg proxy when service provider has two registrars

I am experimenting with using openser as the basis for a home telephone system. Openser resides on the same node as my NAT. I have also employed the rtp proxy feature. I have had some success with my setup but recently I encountered a problem registering with a particular service provider. My phones are set up to use openser as an outgoing proxy. They can register with various service providers except one. From tracing the messages I believe I know what is going wrong. The REGISTER is sent to openser which does a DNS lookup. This results in two registrars. Openser picks one and forwards the messages. The registrar issues a challenge as expected which is sent back to the phone. The phone sends its credentials to openser. Openser does another DNS lookup and sends the message to the other registrar. This registrar also issues a challenge. Thus it continues forever with the credentials being sent to the wrong registrar. My question is, how do I get openser to send the credentials to the registrar that is requesting them? Thanks in advance for any advice.