SIP-T41P losing registration

[azurtem]

Hi

My client has eleven agencies in the West Indies, and each is equipped with an Mikrotik RB951G-2HnD router

Each router has two uplinks, an SDSL and an ADSL link

The VoIP phones are Yealink SIP-T41P (36.72.0.2) and DECT W52P (25.40.0.15)

When connected via the ADSL link the phones frequently lose their registration status and can't be reached

I have to login to the RB951G and remove their tracked connections in the Firewall tab to resolve this

For now I have scheduled a script to reset the connections to port 5060 every ten minutes

I would like to monitor the phones to detect when the registration is lost

Any suggestions on how to monitor this status ?
SNMP doesn't seem to be an option since we can only interrogate static data

thanks
yann