pua_bla/notify.c requires the Subscription-State header value "active" to have an "expires" parameter and throws an error if it's not present.
RFC3265 (the important section listed below indicates that the "expires" is optional by use of the conditional "If".
3.2.4. Subscriber NOTIFY Behavior
... If the header also contains an "expires" parameter, the subscriber ...
Note that Polycom phones appear to contain an "expires" parameter while Aastra phones to not.