Sprint and T-Mobile are calling into question AT&T's recently approved IP-network trials in Florida and Alabama, saying the initiative is holding back the rest of the industry's move to develop cross-carrier IP interconnections.

"AT&T’s proposed experiment is putting the cart before the horse," Sprint wrote in a filing, arguing that many of the benefits of AT&T's transition to an all-IP infrastructure will be lost if carriers do not first migrate their wholesale and inter-carrier interconnections to IP.

Calling AT&T's experiment "isolated" and "complicated", Sprint argued that "carriers should not have to wait for this experiment to finish before migrating their networks from Time Division Multiplex (TDM) to IP or to interconnect in IP format with other carriers. 

T-Mobile expressed similar reservations in a separate filing, urging the commission not to let trials such as AT&T’s distract it from ensuring interconnection amongst all carriers. 

T-Mobile suggested that the commission ensure AT&T’s trials do not involve "unnecessary, inefficient, and consumer-impacting TDM-IP conversions or needlessly inefficient" Points of Interconnection (POI). 

"The trial will not test important enduring values of consumer protection and competition if other carriers that already operate IP networks are required to exchange traffic with AT&T’s trials in TDM format and at inefficient POIs," T-Mobile wrote. 

T-Mobile also suggested that the data from AT&T's trials will be of limited utility given that AT&T "certainly will moderate its behavior during the trial, given its desire to remove regulatory constraints post transition."

AT&T has committed to phasing out its copper networks for an all-IP infrastructure by 2020. The carrier expects to see big cost savings by shutting down its legacy networks.