I have a few ideas.. But before making suggestions on what to try..
You are being contracted by the VoIP company to fix this issue?
said by azurtem:The sites usually have both an SDSL and an ADSL
link.
Does this particular site have both?
said by azurtem:My client believes that the phone's DNS lookups
could be to blame for this situation.
Why does your client believe this?
If that is the case, hardcode the IP into the phones.
What is the Mikrotik config? Which firmware version?
said by azurtem:In the present case they register and stay registered according to the registration frequency setting, until
the point at which they just stop registering; which could be a few hours later.
Do you or your client have a syslog server? What do the logs indicate? (Both phone and server side)
said by azurtem:I on the other hand think that maybe it's because
at some point the trafic sent is responded to via
the same uplink.
Huh?
said by azurtem:In the present case they register and stay registered according to the registration frequency setting, until the point at which they just stop registering; which could be a few hours later.
What do they do to get the phones to re-register?
Can the not-registered phones still make outgoing calls?
What model(s) of phones? Which firmware?
Are all the sites the same?
What is different about this site compared to the others?
Lastly, if the SDSL is just for the VoIP, why not have a separate router for the VoIP, so not needing to do dual-WAN configuration.