Cisco 7960 has issues on some DSLs?

[robertdm1973]

Hi-

We have an AT&T DSL on our remote office. When I tried connecting the 7960 directly behind the DSL modem (AT&T provided) my 7960 has either problems registering or getting kicked-out by the server after a few minutes.

But when I installed a Linksys router/4-port switch in between them, and connect the same phone to it instead of directly to the AT&T modem, the problem goes away.

Does anyone knows what happened here?

Thanks.

[robertdm1973]

I forgot to mention that we have a third-party VOIP provider but we are the ones supplying the Cisco 7960 phones.