Trying to get property of non-object warnings

Noticed while testing Voip Tropo in D7 but probably affects D6 and other servers as well:

Notice: Trying to get property of non-object in VoipSession::getSessionByNumber() (line 25 of /data/var/www/tamer_d7/sites/all/modules/voipdrupal/modules/voipcall/voipsession.inc).

Notice: Trying to get property of non-object in VoipCall->getUid() (line 245 of /data/var/www/tamer_d7/sites/all/modules/voipdrupal/modules/voipcall/voipcall.inc).

Notice: Trying to get property of non-object in VoipSession::getSessionByNumber() (line 27 of /data/var/www/tamer_d7/sites/all/modules/voipdrupal/modules/voipcall/voipsession.inc).